Nitride semiconductor element

ABSTRACT

In a nitride semiconductor device having an active layer  12  between a first electrically conductive type of layer and a second electrically conductive type of layer, a quantum well structure is adopted in which an active layer  12  has at least a well layer  11  formed of a nitride semiconductor containing In and Al and a barrier layer  2  formed of a nitride semiconductor containing Al, whereby a laser device excellent in emitting efficacy at a short wavelength region is obtained. It is particularly preferable that said well layer  1  is formed of Al x In y Ga 1−x−y N (0&lt;x≦1&lt;0&lt;y≦1, x+y&lt;1) and said barrier layer  2  is formed of Al u In v Ga 1−u−v N (0&lt;u≦1, 0≦v≦1, u+v&lt;1). 
     Such a light emitting device is realized to obtain excellent efficacy in emitting light of short wavelength in a region of 380 nm.

TECHNICAL FIELD

The present invention relates to a nitride semiconductor device whichuses III–V Group nitride semiconductor, used in a light emitting devicesuch as a light emitting diode device (LED), a laser diode device(LD),super photoluminescent diode and the like, a light receiving device suchas a solar battery, a photosensor and the like, or an electronic devicesuch as a transistor, a power device and the like, in particular, anitride semiconductor light emitting device operable to emit light ofwavelength of 380 nm or shorter.

BACKGROUND

Currently, a semiconductor laser using a nitride semiconductor has anincreased demand on utilization of an optical disk system which canrecord and reproduce information at a large capacity and a high density.For this reason, a semiconductor laser device using a nitridesemiconductor has been intensively studied. In addition, it is thoughtthat a semiconductor laser device and a light emitting device using anitride semiconductor can be oscillated at a wide region fromultraviolet to red color. Therefore, it is applied not only to a lightsource for the aforementioned optical disk system but also to a lightsource for a laser printer and an optical network. The present applicanthave already announced a laser which can continuously oscillate forlonger than ten thousands hours under the conditions of 405 nm, roomtemperature and 5 mW.

In addition, a laser device, a light emitting device and a lightreceiving device using a nitride semiconductor have a structure in whichan active layer may be formed by using a nitride semiconductorcontaining In and, therefore, formation of a more excellent activeregion in an active layer becomes important for improving the deviceproperties.

In addition, in a nitride semiconductor device, particularly, in a laserdevice and a light emitting device, light emitting and oscillation at awavelength of 380 nm or shorter are highly required. In theaforementioned optical disk system, a recording density is improved by ashorter wavelength and, in a light emitting device, a nitridesemiconductor device becomes important as a light source for exciting afluorescent body. Also in other applications, many uses can be realizedby a light source of further shorter wavelength.

In order to obtain light emitting at a short wavelength in a nitridesemiconductor laser device or light emitting device, an emittingwavelength can be varied by changing an In crystal mixing ratio in anitride semiconductor containing In in an active layer or a lightemitting layer and more specifically, an emitting wavelength can beshortened by decreasing an In crystal mixing ratio. In addition, when anactive layer has a structure in which said active layer is sandwichedbetween an upper cladding layer and a lower cladding layer in an endemitting device or a laser device, by rendering a refractive index ofboth cladding layers small and rendering a refractive index of inside ofa waveguide between an upper cladding layer and a lower cladding layerhigh, the light is effectively confined in a waveguide, which results incontribution to decrease in a threshold current density in a laserdevice.

However, as a wavelength grows shorter, it becomes difficult to use aquantum well structure of InGaN or InGaN/InGaN which have previouslybeen used as a light emitting layer and, at a wavelength of not greaterthan 365 nm corresponding to a band gap for GaN, it becomes difficult touse InGaN as a light emitting layer. In addition, when a wavelengthbecomes shorter, the loss occurs due to light absorption in a guidinglayer in a waveguide, resulting in an enhanced threshold current.Further, also in light confinement by an upper cladding layer and alower cladding layer, since the use of GaN maintains a difference in arefractive index for loss due to light absorption and light confinementin a waveguide, it is necessary to use a nitride semiconductor having agreat Al ratio and, thus, a problem of the crystallizing propertybecomes more important.

In addition, as an attempt to make a wavelength of such the nitridesemiconductor device shorter, a quantum well structure of AlGaN/AlGaN isused and, however, there is a tendency that sufficient output can not beobtained as compared with the conventional InGaN system.

In addition, in the case where a nitride semiconductor containing Alsuch as AlGaN is used in an device, a difference in the thermalexpansion coefficient and the elasticity are greatly different ascompared with other nitride semiconductors containing no Al and, thus,when a nitride semiconductor containing Al is used, crack is easilyproduced, production of crack destroys an device unlike the othercrystallizing properties and, therefore, if crack is not prevented fromoccurring, an device does not serve as a nitride semiconductor device.For this reason, in a light emitting device and a laser device using theaforementioned active layer having an emitting wavelength of 380 nm orshorter, since a nitride semiconductor containing Al can make the bandgap energy greater in a nitride semiconductor, it is used in an activelayer, as well as a carrier confining layer, a light guiding layer and alight confining layer having the greater band gap energy than that ofthe active layer. That is, in the aforementioned light emitting deviceof a shorter wavelength area, a nitride semiconductor containing Al hasa multi-layered structure. On the other hand, the aforementioned problemof production of crack becomes serious and, therefore, there is atendency that shorter wavelength and prevention of crack production hasthe contradictory relationship, and this becomes a serious barrier tomore shorter wavelength in a light emitting device of a nitridesemiconductor. Further, since GaN has an absorption end for light at 360nm and has a high absorption coefficient even at a region of a longerwavelength than that of the end by around 10 nm, it becomes difficult touse GaN in a light emitting device and a laser device in theaforementioned shorter wavelength area of 380 nm or shorter.

In addition, since an active layer in a light emitting device or a laserdevice has the emitting efficacy and the internal quantum efficacydepending greatly upon the crystallizing property thereof as describedabove, the crystallizing property of an electrically conductive type oflayer arranged below an active layer becomes an extremely importantfactor for improving the properties of an device. Usually, a nitridesemiconductor light emitting device has a structure in which a n-typelayer, an active layer and a p-type layer are laminated in this orderand, in this case, it is necessary to make the crystallizing property ofa n-type layer better. On the other hand, as described above, there is atendency that the crystallizing property of a nitride semiconductorcontaining Al is greatly deteriorated as compared with other nitridesemiconductors containing no Al and, previously, for the purpose ofavoiding such the problem, a nitride semiconductor layer containing Inis used as a substrate layer for a nitride semiconductor containing Alto alleviate occurrence of the internal stress due to a difference inthe thermal expansion coefficient, a nitride semiconductor containing noAl such as Ga is provided adjacent to a nitride semiconductor layercontaining Al to realize recovery of the crystallizing property andalleviation of the internal stress, whereby, it allows an device such asa laser device having a structure in which a nitride semiconductor layercontaining Al is provided therein, to be practically exerted. However,in the aforementioned light emitting device and laser device having ashorter wavelength, a nitride semiconductor containing no Al becomes alight-absorbing layer and use of it in an device structure is notpreferable and, for this reason, most of device structures use a nitridesemiconductor layer containing Al. Thus, a light emitting device and alaser device having the practical threshold Vf and emitting efficacy cannot be obtained due to the aforementioned crystallizing property andoccurrence of crack and, in particular, in a laser device which usesmuch nitride semiconductors containing Al and having a greater Alcrystal mixing ratio in a light-guiding layer or a cladding layer forlight confinement, a laser device which can be continuously oscillatedat room temperature can not be obtained.

SUMMARY OF THE INVENTION

An object of the present invention is to increase emitting output in anitride semiconductor device, more specifically, in a laser device or alight emitting device having a light wavelength of 380 nm or shorter, tosuppress light absorption low in a waveguide in which an active layerhas a decreased threshold current density and is provided betweencladding layers, to confine the light effectively into a waveguideincluding an active layer, and to form a device structure with thebetter crystallizing property.

An another object of the present invention is to elucidate a cause for aproblem that a particularly remarkable increase in a threshold appearsat laser oscillation at 380 nm or shorter and provide the means forsolving the problem.

In view of the aforementioned circumstances, the present invention canobtain a nitride semiconductor device which is excellent in the deviceproperties such as a threshold current density, has the bettercrystallizing property, is excellent in emitting output, and can realizea shorter wavelength.

That is, a nitride semiconductor device of the present invention canattain the above object by the following features.

(1) A nitride semiconductor device, which comprises an active layerprovided between a first electrically conductive type of layer and asecond electrically conductive type of layer, wherein said active layerhas a quantum well structure including at least a well layer formed of anitride semiconductor containing In and Al, and a barrier layer formedof a nitride semiconductor containing Al.

Whereby, inclusion of In in a well layer improves the emitting efficacyand, on the other hand, variation of an Al ratio, a desired emittingwavelength corresponding to a band gap energy thereof can be obtained,resulting in a laser device or a light emitting device excellent in theemitting efficacy and the internal quantum efficacy. In addition, byallowing a barrier layer to contain at least Al, an active layer havinga quantum well structure can be realized in which the band gap energythereof is larger than that of a well layer so as to adjust an emittingwavelength and, thus, an active layer having the excellent deviceproperties at a short wavelength region of a wavelength of 380 nm orshorter is obtained.

(2) It is preferable in the above feature that said well layer is formedof Al_(x)In_(y)Ga_(1−x−y)N (0<x≦1, 0<y≦1, x+y<1) and said barrier layeris formed of Al_(u)In_(v)Ga_(1−u−v)N (0<u≦1, 0≦v≦1, u+v<1).

By formation of a well layer of InAlGaN quaternary mixed crystal, thenumber of constituent elements is minimized and deterioration of thecrystallizing property is suppressed, resulting in a well layer and anactive layer having the high emitting efficacy. For this reason, 0<x<1,and 0<y<1 is preferable. In addition, formation of a barrier layer ofAlGaN or InAlGaN, a quantum well structure is formed in which a desireddifference in the band gap energy is set between a well layer and abarrier layer. On the other hand, by using the same constituent elementsas those of a well layer or by rendering smaller, the crystallizingproperty in an active layer can be maintained better. Preferably, x<ucan afford an active layer having the excellent crystallizing property.

(3) It is preferable in the above feature that a thickness of said welllayer is smaller than that of the barrier layer.

Whereby, carriers can be injected efficiently in an active layer,resulting in a quantum well structure excellent in the emittingefficacy. In particular, by adopting a thickness of an n-side barrierlayer arranged within an active layer in a side nearest to an n-typenitride semiconductor layer greater than a thickness of a well layer, athickness of other barrier layer, especially, a thickness of a barrierlayer between well layers , p-type carriers can be effectively confinedinto an active layer. Preferably, by rendering a thickness of an n-sidebarrier layer 10 nm or greater, it functions as an excellenthole-confinement layer, resulting in an active layer having the betterproperties.

(4) It is preferable in the above feature that an In composition ratio yin said well layer is in a range of not less than 0.02 and not greaterthan 0.05.

Whereby, by adopting y of 0.02 or greater, a well layer and an activelayer excellent in the emitting efficacy and the internal quantumefficacy can be obtained. By adopting y of 0.05 or smaller, an activelayer can be obtained in which deterioration of the crystallizingproperty is suppressed in a mixed crystal system containing In and Al.By adopting y of a range 0.02 to 0.05, threshold current density can bemaintained lower.

(5) It is preferable in the above feature that an In composition ratio yin said well layer is in a range of not less than 0.03 and not greaterthan 0.05.

Whereby, by adopting y of 0.03 or greater, a well layer and an activelayer excellent in the emitting efficacy and the internal quantumefficacy can be obtained. By adopting 0.05 or smaller, an active layercan be obtained in which deterioration of the crystallizing property issuppressed in a mixed crystal system containing In and Al.

(6) It is preferable in the above feature that an emitting wavelength ofsaid active layer is 380 nm or shorter.

The aforementioned active layer structures can afford a device havingthe excellent properties in a short wavelength region of 380 nm orshorter.

(7) It is preferable in the above feature that said device has a laserdevice structure in which said first electrically conductive type oflayer has a first light guiding layer, said second electricallyconductive type of layer has a second light guiding layer, and saidactive layer is provided between said first light guiding layer and saidsecond light guiding layer, and

the band gap energies E_(g) of both said first light guiding layer andsaid second light guiding layer are greater than the photon energy E_(p)of laser light by 0.05 eV or greater (E_(g)−E_(p)≧0.05 eV).

Whereby, in a laser device and an end-emitting type device, a waveguideexcellent in guiding the light can be obtained. More preferably,E_(g)−E_(p)≧0.1 can form the further better waveguide in theaforementioned short wavelength, resulting in improvement in the deviceproperties.

(8) It is preferable in the above feature that said first light guidinglayer and/or said second light guiding layer are formed ofAl_(x)Ga_(1−x)N (0≦x≦1).

Whereby, a waveguide structure with low light loss in a short wavelengthregion can be obtained, which results in improvement in the propertiesof a laser device and an end emitting device.

(9) It is preferable in the above feature that said active layer emittslight of wavelength of 380 nm or shorter, and said first electricallyconductive type of layer and/or said second electrically conductive typeof layer are formed of Al_(x)Ga_(1−x)N (0<x≦1).

For example, by forming a cladding layer of AlGaN, carrier confinementand light confinement can be realized better. When a light guiding layeris provided between a cladding layer and an active layer, by varying anAl ratio among these of the cladding layer and the light guiding layerto set a desired difference in a refractive index between both layers, alaser device and an end emitting device excellent in the properties areobtained.

(10) A nitride semiconductor device having an active layer between afirst electrically conductive type of layer and a second electricallyconductive ype of layer, wherein said active layer has a quantum wellstructure including at least a well layer formed of a nitridesemiconductor containing Al, and a first barrier layer formed of anitride semiconductor having a band gap energy larger than that of thewell layer in a side near to the first electrically conductive type oflayer from the well layer, and

said first electrically conductive type of layer includes a firstnitride semiconductor layer having a band gap energy smaller than thatof said first barrier layer, and said first nitride semiconductor layeris provided near said first barrier layer.

In the conventional AlGaN series active layer, the band gap energylarger than that of a well layer is usually required in respectiveelectrically conductive type of layers provided in both sides of theactive layer as a carrier injecting layer. However, in thisconstruction, by providing at a first electrically conductive type oflayer a first nitride semiconductor layer having a band gap energysmaller than that of a first barrier layer within an active layer, anovel device structure can be realized, which forms an active layerhaving the better crystallizing property and which has the function ofconfinement of carriers from a second electrically conductive type oflayer into a well layer with the first barrier layer.

For a well layer, a nitride semiconductor containing Al having the bandgap energy which is at least the same as that of GaN or larger may beused and, specifically, the aforementioned composition may be used. Alsofor a first barrier layer, a nitride semiconductor having theaforementioned composition may be used.

For a first nitride semiconductor layer, by using preferably a nitridesemiconductor having the band gap energy larger than that of the welllayer, it functions as a layer for effectively injecting carriers intoan active layer and a well layer. More specifically, a nitridesemiconductor containing Al may be used, and an active layer having thepreferable crystallizing property can be formed by using preferablyAl_(x)Ga_(1−x)N (0≦x<1).

(11) It is preferable in the above feature that said first barrier isarranged in a side nearest to the first electrically conductive type oflayer within the active layer, and said first nitride semiconductorlayer is contacted with the active layer.

Whereby, by arranging a first barrier layer in a side near to a firstelectrically conductive type of layer, that is, nearest to a firstelectrically conductive type of layer than other barrier layers withinan active layer, as described above, a first barrier layer suitablyfunctions as a carrier confining layer in a side of a first electricallyconductive type of layer and, thus, the emitting efficiency in a welllayer can be enhanced. Upon this, preferably, arrangement at themostouter side of an active layer and nearest to a first electricallyconductive type of layer can permit the carrier confinement functionaforementioned.

(12) It is preferable in the above feature that said first electricallyconductive type of layer is of n-type, and said second electricallyconductive type of layer is of p-type.

That is, by this feature, the aforementioned first barrier layerfunctions for confinement of holes, whereby, in the aforementioned firstnitride semiconductor layer, since the barrier layer can function toinject electrons (carriers of first electrically conductive type) into afirst electrically conductive type of layer and, on the other hand,functions for confinement of holes (carriers of second electricallyconductive type) with difficulty, an active layer structure can beobtained which can realize confinement of holes by a first barrierlayer.

(13) It is preferable in the above feature that an Al crystal mixingratio X_(B1) in said first barrier layer and an Al crystal mixing ratioX_(w) in the well layer satisfy the following relation:X_(B1)−X_(W)≧0.05.

That is, by setting an Al crystal mixing ratio by the aforementionedequational relationship, a first barrier layer can suitably function forconfinement of carriers (preferably holes) of a second electricallyconductive type. More preferably, by X_(B1)−X_(w)≧0.1, offset (potentialbarrier) can be formed which sufficiently functions for theaforementioned carrier confinement. Upon this, an upper limit of adifference in an Al crystal mixing ratio (X_(B1)−X_(w)) is not limitedin the confinement effects, but considering the crystallizing property,0.5 or smaller is preferable. More preferably, by an upper limit of 0.3or smaller, a device structure containing an active layer and a firstelectrically conductive type of layer can be formed with the bettercrystallizing property and sufficient light confinement.

(14) It is preferable in the above feature that a thickness of saidfirst barrier layer is 30 Å or greater.

By this feature, a first barrier layer can have a thickness allowingcarrier confinement. Preferably, by a thickness of 50 Å or greater, astructure with enhanced confinement efficacy can be obtained because,when a thickness of a first barrier layer is small, the tunnelingphenomenon for carriers occurs as shown in FIG. 14B. By a thickness of50 Å or greater, the confinement efficacy can be enhanced. An upperlimit is not particularly limited in the confinement efficacy, butconsidering the crystallizing property, an upper limit is 300 Å orsmaller. In order to obtain the more preferable crystallizing property,an upper limit is 200 Å or smaller. When an upper limit is 150 Å orsmaller, a first barrier layer can be obtained which has the bettercrystallizing property and suitably suppresses tunneling effect.

(15) It is preferable in the above feature that a waveguide isconstructed of a pair of light guiding layers and an active layertherebetween, and

the light guiding layer is provided in the first electrically conductivetype of layer, and has said first nitride semiconductor layer.

By allowing the aforementioned nitride semiconductor layer to functionas a light guiding layer or a part thereof, even when a guiding layerhaving a necessary thickness for light guiding is formed, by using afirst nitride semiconductor layer in which an Al crystal mixing ratio issuppressed, an active layer may be formed with the better crystallizingproperty. Examples of a light emitting device having such the waveguideinclude a laser device, an end emitting device and super luminescentdiode.

(16) It is preferable in the above feature that said device is operableto oscillate light of wavelength of 375 nm or shorter,

an Al_(x)Ga_(1−x)N quantum well layer (x≧0) is provided between barrierlayers which are formed of Al_(y)In_(z)Ga_(1−y−z)N (z≧0), and

a band gap energy E_(w) of the well layer is larger than a band gapenergy E_(b) of the barrier layer by 0.2 eV or greater.

When a quantum well layer is GaN, it is preferable to determine thecomposition of Al_(y)Ga_(1−y)N and that of Al_(y)In_(z)Ga_(1−y−z)N byconsidering the aforementioned difference in band gap. Also when aquantum well layer is AlGaN, it is preferable to determine thecomposition of Al_(y)Ga_(1−y)N and that of Al_(y)In_(z)Ga_(1−y−z)N byconsidering the aforementioned difference in band gap.

Although an active layer may be formed only of the aforementionedquantum well layer, a single quantum well layer may be formed byarranging the barrier layers both sides of a quantum well layer.

When a multiple quantum well layer is formed, an active layer is formedby combining a quantum well layer and a barrier layer, and a final layermay be formed of a well layer or a barrier layer. Adjustment can beperformed depending upon the relationship with a layer structure(capping layer, guiding layer, cladding layer) connected to an activelayer. Although the number of layers of a multiple quantum well issufficiently around 2 or 3, the number of well layers may be increasedin such a range that the internal quantum efficacy in an active layer isnot suppressed. Alternatively, since a recombination rate in a welllayer in a region near a p-side is high in many cases, variation may bemade so that a region near a p-side may be a non-doped layer and a welllayer in a region near n-side may be doped with silicon or the like.

(17) It is preferable in the above feature that a thickness of the welllayer is 300 Å or smaller, preferably 200 Å or smaller.

(18) It is preferable in the above feature that a thickness of thebarrier layer is 300 Å or smaller, preferably 200 Å or smaller.

(19) It is preferable in the above feature that said device has a SCH(separate confinement hetero) structure in which a light guiding layerand a cladding layer are provided apart from each other, and the bandgap energy E_(g) of the guiding layer is greater than the photon energyE_(p) at oscillation by 0.05 eV.

(20) It is preferable in the above feature that the light guiding layercomprises an Al_(a)Ga_(1−a)N/Al_(b)Ga_(1−b)N (a≠b) superlattice layers.

(21) It is preferable in the above feature that the cladding layercomprises an Al_(c)Ga_(1−c)N/Al_(d)Ga_(1−d)N (c≠d) superlattice layers,and the band gap energy E_(c) of the cladding layer is greater than thatof the light guiding layer.

(22) It is preferable in the above feature that said device has a GRIN(graded index) structure in which a light confining layer with astepwise varying refractive index is formed outside the quantum welllayer, and non-doped layers are provided above and below the activelayer. This GRIN structure may be used together with the aforementionedSCH to obtain a GRIN-SCH structure.

In addition, another aspect of the present invention will be explainedbelow and this aspect may be used by combining with any one of theaforementioned features.

#2{Means to Solve the Problem]

One aspect thereof is an AlGaN series semiconductor light emittingdevice, which comprises an Al_(x)Ga_(1−x)N quantum well layer (x≧0) asan active layer formed on a GaN substrate, wherein said device can beoscillated at 375 nm or shorter. Another aspect is to obtain a devicestructure which can be also used in a light emitting device and a laserdevice having a waveguide, at a shorter wavelength, which can avoid aproblem of occurrence of crack.

A GaN substrate can generally decrease a defect density of an epitaxiallayer formed thereon in comparison with a sapphire substrate or asilicon carbide substrate. In particular, it is preferable that thecrystal defect of a GaN substrate is 10⁷/cm² or smaller, further10⁴/cm². Upon this, the crystal defect is mainly due to penetrationdislocation, a light emitting device in which a waveguide or a currentinjection region is formed in this less defective region is preferable.This value of crystal defect density or lower can manufacture a laserdevice having no defect in a waveguide.

As used herein, a GaN substrate means a substrate in which GaN can bevapor phase-grown by MOCVD(metal oxide chemical vapor phasedeposition)or MBE(molecular beam epitaxy) method, such as a GaNsubstrate formed by laterally growing, by selective growth, GaN on aheterogenous substrate such as a sapphire substrate or the likeaccording to ELO (epitaxial lateral overgrowth) method (for example,comprising a GaN layer formed by laterally growing GaN by growthselective for a plurality of SiO₂ stripe regions formed on a GaN layerand GaN regions exposed between those regions), a GaN substrate obtainedby laminating and vapor phase-growing a GaN layer on this substrate byHVPE(halide vapor phase epitaxy) method, or a combination with the MOCVDmethod, a GaN substrate obtained by vapor phase-growing GaN on thissubstrate according to the aforementioned ELO method, a GaN substrateformed by recrystallizing GaN on a GaN seed crystal in a supercriticalfluid NH₃ and the like.

In the case where a GaN substrate is a GaN substrate formed by utilizingthe aforementioned ELO method and a light emitting device formed on thesubstrate is a ridge type semiconductor laser, it is preferable that awaveguide formed from the aforementioned quantum well layer is formedparallel with the aforementioned SiO₂ stripe, because the crystaldefects are developed by concentration parallel to SiO₂ stripes and forma less dense stripe shape and, therefore, the defect in a waveguide canbe excluded by forming a waveguide in a region having the low defectdensity.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view explaining a laser devicestructure according to one embodiment of the present invention.

FIG. 2A is a schematic cross-sectional view explaining a laminatedstructure relating to one embodiment of the present invention and FIG.2B is a view explaining the relationship between each layer and an Alratio.

FIG. 3A is a schematic view explaining a laminated structure of a deviceaccording to one embodiment of the present invention and FIG. 3B is anenergy band view thereof.

FIG. 4 is a schematic view explaining an energy band according to oneembodiment of the present invention.

FIG. 5 is a schematic view explaining an energy band according to oneembodiment of the present invention.

FIG. 6A is a schematic view explaining an energy band according to oneembodiment of the present invention, and FIGS. 6B to 6D show change in adoping amount of each electrically conductive-type impurity (dopant).

FIG. 7 is a schematic cross-sectional view explaining a laminatedstructure of an active layer according to one embodiment of the presentinvention.

FIG. 8 is a schematic cross-sectional view explaining a device structureaccording to one embodiment of the present invention.

FIG. 9A is a schematic view explaining the relationship between an Inratio and emitting efficacy, and FIG. 9B is a schematic view explainingthe relationship between an In ratio and a threshold current density, inan active layer according to the present invention.

FIG. 10 is a schematic view explaining dependency of an Al crystalmixing ratio on a threshold current density and a wavelength under pulseoscillation, in an active layer according to the present invention.

FIG. 11 is a schematic view explaining dependency of an In crystalmixing ratio on a threshold current density and a wavelength under pulseoscillation, in an active layer according to the present invention.

FIG. 12A is a schematic cross-sectional view explaining a laminatedstructure relating to one embodiment of the present invention and FIG.12B is a schematic view explaining a band structure in the bias statuscorresponding to the laminated structure.

FIGS. 13A and 13B are schematic views explaining band structures in thebias status of a laser device of the prior art.

FIGS. 14A and 14B are schematic views explaining band structures in thebias status in an device relating to one embodiment of the presentinvention.

FIG. 15A is a schematic cross-sectional view explaining a laminatedstructure of a light emitting device relating to one embodiment of thepresent invention and FIG. 15B is a view explaining the relationshipbetween each layer and an Al ratio.

FIG. 16 is a view explaining the relationship with an Al ratio of eachlayer corresponding to a laminated structure of a laser device of theprior art.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A nitride semiconductor used in a nitride semiconductor device of thepresent invention is III–V Group nitride semiconductor(In_(α)Al_(β)Ga_(1−α−β)N, 0≦α, 0≦β, α+β≦1) such as GaN, AlN or InN, or amixed crystal thereof. Besides, a mixed crystal may be used in whichB(boron) is used as a III Group element, or a part of N(nitrogen) as a VGroup element is substituted with P(phosphorous) or As(arsenic). Inaddition, a nitride semiconductor containing Al has β≧0, and a nitridesemiconductor containing In has α>0.

In addition, as n-type impurity used in a nitride semiconductor layer,IV Group elements or VI Group elements such as Si(silicon),Ge(germanium), Sn(tin), S(sulfur), O(oxygen), Ti(titanium),Zr(zirconium) and the like can be used. Preferably, Si, Ge and Sn areused and, most preferably, Si is used. In addition, a p-type impurity isnot particularly limited but examples thereof include Be(bertllium),Zn(zinc), Mn(manganese), Cr(chromium), Mg(magnesium) and Ca(calcium).Preferably, Mg is used. Whereby, each electrically conductive type ofnitride semiconductor layer is formed, constituting each electricallyconductive type of layer to be described later.

[Embodiment 1A (Quantum Well Structure)]

A nitride semiconductor device of the present invention has a structurein which an active layer is provided at least between a firstelectrically conductive type of layer and a second electricallyconductive type of layer. The nitride semiconductor device of thepresent invention will be explained in detail below.

(Active Layer)

An active layer in the present invention has a quantum well structure,has a well layer formed of a nitride semiconductor containing at leastIn(indium) and Al(aluminum), and a barrier layer formed of a nitridesemiconductor containing Al. In addition, an active layer emits lightof, preferably a short wavelength of 380 nm or shorter and,specifically, the aforementioned well layer has the band gap energy of awavelength of 380 nm or shorter. Upon this, a nitride semiconductor usedin the active layer may be either of undoped, n-type impurity doped orp-type impurity doped. Preferably, by provision of non-doped or undoped,or n-type impurity doped nitride semiconductor in an active layer, highoutput is attained in a nitride semiconductor device such as a laserdevice and a light emitting device. Preferably, by rendering a welllayer undoped and rendering a barrier layer n-type impurity doped, alaser device and a light emitting device having high output areobtained, resulting in a device having the high emitting efficacy. Here,a quantum well structure may be a multiple quantum well structure or asingle quantum well structure. Preferably, by adopting a multiplequantum well structure, improvement in output and decrease in anoscillating threshold becomes possible. As a quantum well structure foran active layer, a structure may be used in which at least one layer ofthe aforementioned well layer and at least one layer of theaforementioned barrier layer are laminated. Upon this, in the case of aquantum well structure, by adopting the number of well layers of notless than 1 and not greater than 4, decrease in a threshold current canbe decreased, for example, in a laser device and a light emittingdevice, being preferable. More preferably, by adopting a multiplequantum well structure having the number of well layers of 2 or 3, highoutput laser device and light emitting device are obtained.

(Well Layer)

In a well layer in the present invention, it is preferable that anitride semiconductor containing In and Al is used, and at least onewell layer comprising a nitride semiconductor containing In and Al iscontained in an active layer. In a multiple quantum well structure,preferably, by adopting such a well layer that all well layers areformed of a nitride semiconductor containing In and Al, a shorterwavelength is obtained, resulting in a light emitting device and a laserdevice having the high output and efficacy. When light emitting spectrumhas an almost single peak, this construction is preferable. On the otherhand, in a multi-color light emitting device having a plurality ofpeaks, by adopting at least one well layer comprising the aforementionednitride semiconductor containing In and Al, an emitting peak at ashorter wavelength region can be obtained and, thus, a light emittingapparatus in a combination with a light emitting device having a varietyof emitting colors or a fluorescent body which is excited at the shorterwavelength region, can be obtained. Upon this, in the case of a deviceof multicolor emitting, by using In_(α)Ga_(1−α)N (0<α≦1) as a specificcomposition for a well layer, a well layer is obtained which can emitand oscillate from ultraviolet to visible light area. Upon this, anemitting wavelength can be determined by an In crystal mixing ratio.

A well layer formed of a nitride semiconductor containing In and Al ofthe present invention affords a wavelength region which is difficult inthe conventional InGaN well layer, specifically, a wavelength of around365 nm corresponding to the band gap energy of GaN, or a shorterwavelength. In particular, the well layer is a well layer having theband gap energy at which light emitting and oscillation are possible ata wavelength of 380 nm or shorter. In the conventional InGaN well layer,at a wavelength around 365 nm corresponding the band gap energy of GaN,for example, at 370 nm, it is necessary to adjust an In ratio at around1% or smaller. When an In ratio grows extremely small like this, theemitting efficacy is decreased, and a light emitting device and a laserdevice having the sufficient output are difficult to be obtained. On theother hand, when an In ratio is 1% or smaller, it is also difficult tocontrol the growth thereof. In the present invention, by using a welllayer formed of a nitride semiconductor containing In and Al, at awavelength region of 380 nm at which effective light emitting wasdifficult previously, the band gap energy can be increased by increasingan Al ratio x. On the other hand, by inclusion of In, it can be used ina light emitting device and a laser device having the better internalquantum efficacy and emitting efficacy.

A specific composition of a nitride semiconductor containing In and Alused in a well layer is a composition represented byAl_(x)In_(y)Ga_(1−x−y)N (0<x≦1, 0<y≦1, x+y<1). In a vapor phase growingmethod such as MOCVD and the like used for growing a nitridesemiconductor, when the number of constituent elements is increased, areaction between constituent elements becomes easier to occur. For thisreason, although, pluralization of a quinary or more mixed crystal ispossible using B, P, As, Sb and the like as described above, preferably,by adopting an AlInGaN quaternary mixed crystal, this reaction betweenelements can be prevented and a well layer can be grown with the bettercrystallizing property. By adopting an In ratio y of not less than 0.02,the better emitting efficacy and internal quantum efficacy are realizedas compared with less than 0.02 as described above. Further, by adoptingy≧0.03, since the efficacy is further improved, a light emitting deviceand a laser device having the excellent properties in a well layer at awavelength of 380 nm or shorter are obtained, being preferable. Inaddition, an upper limit of an In ratio is not particularly limited but,by adopting y≦0.1, deterioration of the crystallizing property due toinclusion of In is suppressed. More preferably, by adopting y≦0.05, awell layer can be formed without deteriorating the crystallizingproperty and, thus, when a plurality of well layers are provided as in amultiple quantum well structure, the crystallizing properties of eachwell layer can be better. Therefore, an In ratio y is preferably in arange of not less than 0.02 and not greater than 0.1, more preferably ina range of not less than 0.03 and not greater than 0.05, which can beapplied to the aforementioned InAlGaN quaternary mixed crystal, beingpreferable. Here, an Al ratio x is not particularly limited but, byvarying an Al ratio, the desired band gap energy and wavelength can beobtained.

In a well layer formed of Al_(x)In_(y)Ga_(1−x−y)N (0<x≦1, 0<y≦1, x+y<1)in the present invention, each property is greatly changed in a range ofan In ratio y in a nitride semiconductor of 0 to 0.1 as shown in FIGS.9A and 9B. As shown in FIG. 9A, the emitting efficacy is greatlyincreased from an In ratio y of around 0.02, and a mild falling curve isshown from around 0.05. On the other hand, in a threshold currentdensity J_(th), a mild falling curve is shown from around 0.02, aminimum value exists in a range of 0.03 to 0.05 and, at an areaexceeding 0.05, a rapid increasing curve is shown. Here, FIGS. 9A and 9Bshow tendency of each property qualitatively in a well layer ofAl_(x)In_(y)Ga_(1−x−y)N (0<x≦1, 0<y≦1, x+y<1) and a barrier layer ofAl_(u)In_(v)Ga_(1−u−v)N (0<u≦1, 0≦v≦1, u+v<1), wherein a y-axis is in anarbitrary unit.

In the present invention, preferably, the band gap energy correspondingto a wavelength of 380 nm or shorter is set by a well layer of a nitridesemiconductor containing Al and In. For doing so, an Al ratio X isrendered 0.02 or greater. In addition, in an area of a wavelength of notgreater than 365 nm corresponding to the band gap energy of GaN, byadopting x of 0.05 or greater, the better light emitting and oscillationbecome possible at a short wavelength.

In addition, it is possible to determine a thickness and a number of awell layer arbitrarily. A specific thickness is in a range of not lessthan 1 nm and not greater than 30 nm. When a thickness is less than 1nm, there is a tendency that a well layer does not function well. When athickness exceeds 30 nm, it becomes difficult to grow a nitridesemiconductor containing In and Al with the crystallizing property,resulting in decrease in the device properties. Preferably, by adoptinga range of not less than 2 nm and not greater than 20 nm, a thresholdcurrent density Vf can be reduced. From a viewpoint of the crystalgrowth, when a thickness is 2 nm or greater, a layer having no greatvariation in a thickness and having a relatively uniform membraneproperty is obtained. By adopting 20 nm or smaller, occurrence of thecrystal defect is suppressed low and, the crystal growth becomespossible. More preferably, by adopting a thickness of a well layer ofnot less than 3.5 nm, there is a tendency that high output laser deviceand light emitting device are obtained. This can be thought as follows:By increasing a thickness of a well layer, light emitting recombinationis made with the high emitting efficacy and internal quantum efficacyrelative to injection of a large amount of carriers as in a laser devicewhich is driven by a great current. This is considered to have theeffect in a multiple quantum well structure. In a single quantum wellstructure, by adopting a thickness of 5 nm or greater, the similareffects to those described above can be obtained. The number of welllayers in an active layer is not particularly limited but is 1 or more.Upon this, in the case where the number of well layers is 4 or more,when a thickness of each layer constituting an active layer growslarger, a thickness of a whole active layer is increased, leading toincrease in Vf. Therefore, it is preferable that a thickness of anactive layer is suppressed low by adopting a thickness of a well layerin a range of 10 nm or smaller. In a multiple quantum well structure,among a plurality of well layers, at least one well layer having athickness in the aforementioned range is provided and, preferably, allwell layers are rendered to have a thickness in the aforementionedrange. In addition, a thickness of each well layer may be different orapproximately the same.

A well layer in the present invention may be doped with p-type impurityor n-type impurity, or may be undoped. Preferably, by adopting n-typeimpurity as impurity to be doped in a well layer, it contributes toimprovement in the emitting efficacy. However, there is a tendency thata nitride semiconductor containing In and Al is used in a well layerand, when the impurity concentration grows larger, the crystallizingproperty is deteriorated. Therefore, it is preferable that a well layerhaving the better crystallizing property is obtained by suppressing theimpurity concentration low. Specifically, in order to make thecrystallizing property better maximally, a well layer is grown undoped.Upon this, the impurity concentration is 5×10¹⁶/cm³ or smaller,resulting in a well layer containing substantially no impurity. Inaddition, in the case where a well layer is doped, for example, withn-type impurity, when doped at the n-type impurity concentration in arange of not less than 1×10¹⁸/cm³ and not greater than 5×10¹⁶/cm³,deterioration of the crystallizing property can be suppressed low and,at the same time, the carrier concentration can be increased and, thus,a threshold current density Vf can be reduced. Upon this, by adoptingthe n-type impurity concentration in a well layer approximately the sameas or slightly smaller than that in a barrier layer, there is a tendencythat light emitting recombination in a well layer is promoted andemitting output is improved, being preferable. Upon this, a well layerand a barrier layer may be grown undoped so as to constitute a part ofan active layer. In addition, in a multiple quantum well structure inwhich a plurality of well layers are provided in an active layer, theimpurity concentration in each well layer may be approximately the sameor different.

In particular, in the case where a device is driven by a great current(such as high output LD, high power LED, and superphotoluminescentdiode), when a well layer is undoped and contains substantially non-type impurity, recombination of carriers in a well layer is promotedand light emitting recombination is realized at the high efficacy.Conversely, when a well layer is doped with n-type impurity, since thecarrier concentration is high in a well layer, a probability of lightemitting recombination is conversely reduced, a vicious cycle leading toincrease in a driving current under a constant output occurs, resultingin tendency of decrease in reliance of a device (device life). For thisreason, in such the high output device, the n-type impurityconcentration in a well layer is at least 1×10¹⁸/cm³ or smaller,preferably, is such the concentration that a well layer is undoped orcontains substantially no n-type impurity, a nitride semiconductordevice is obtained which has high output and can be driven stably. Inaddition, a laser device in which a well layer is doped with n-typeimpurity, there is a tendency that a spectrum width of a laser light ata peak wavelength is widened, being not preferable. The concentration is1×10¹⁸/cm³ or smaller, preferably 1×10¹⁷/cm³ or smaller.

(Barrier Layer)

In the present invention, as a composition of a barrier layer, a barrierlayer comprising a nitride semiconductor containing Al is used. Here, inan active layer in the present invention, it is necessary that at leastone barrier layer in an active layer comprises a nitride semiconductorcontaining Al. All barrier layers in an active layer may comprise anitride semiconductor containing Al, or a barrier layer comprising anitride semiconductor containing no Al may be provided in an activelayer. It is necessary that a barrier layer is a nitride semiconductorhaving the greater band gap energy as compared with a well layer. In aregion of an emitting wavelength of 380 nm in a well layer, it ispreferable that a nitride semiconductor containing Al is used in acorresponding barrier layer. As a barrier layer of a nitridesemiconductor containing Al, preferably, a nitride semiconductorrepresented by Al_(u)In_(v)Ga_(1−u−v)N (0<u≦1, 0≦v≦1, u+v<1) is used.Specifically, in a barrier layer of a nitride semiconductor containingAl, an AlInGaN quaternary mixed crystal and AlGaN ternary mixed crystalrepresented by the composition may be used. In addition, an Al ratio Uin a barrier layer is greater than an Al ratio x in a well layer of anitride semiconductor containing Al and In. By adopting u>x and settinga sufficient difference in a band gap energy between a well layer and abarrier layer, a quantum well layer structure is formed having thebetter emitting efficacy, a laser device or a light emitting device. Inaddition, when a barrier layer contains In (v>0), by adopting an Inratio v of preferably not greater than 0.1, deterioration of thecrystallizing property can be suppressed. More preferably, a range ofnot greater than 0.05 can be applied. When an In ratio v exceeds 0.1, areaction between Al and In at growth is promoted, and the crystallizingproperty is deteriorated and, thus, the better membrane is not formed.Further, by adopting v≦0.05, a barrier layer is formed with the furtherbetter crystallizing property. In addition, as described above, a widerIn ratio can be applied to a barrier layer as compared with a welllayer, and the band gap energy difference is set mainly by an Al ratioand, thus, v≧y can be adopted. By adopting such the In ratio, a criticalthickness of a well layer or a barrier layer can be changed, a thicknesscan be set relatively freely in a quantum well structure, and an activelayer having a desired property can be designed.

In addition, in an active layer having a quantum well structure, abarrier layer and a well layer may be formed alternately, or a pluralityof barrier layers may be provided relative to one well layer.Specifically, 2 or more barrier layers held by well layers may beprovided, or a structure may be provided in which a multi-layeredbarrier layer and a well layer are laminated alternately.

In addition, a barrier layer may be doped with p-type impurity or n-typeimpurity, or may be undoped as in the aforementioned well layer.Preferably, a barrier layer is doped with n-type impurity or non-doped,or undoped. Upon this, when a barrier layer is doped, for example,n-type impurity, the concentration is at least 5×10¹⁶/cm³ or greater.Specifically, for example, in the case of LED, a barrier layer hasn-type impurity at a range of not less than 5×10¹⁶/cm³ and not greaterthan 2×10¹⁸/cm³. In addition, in the case of higher output LED and highoutput LED, a barrier layer is preferably doped with a range of not lessthan 5×10¹⁷/cm³ and not greater than 1×10²⁰/cm³, preferably a range ofnot less than 1×10¹⁸/cm³ and not greater than 5×10¹⁹/cm³. When a barrierlayer is doped at the high concentration like this, it is preferablethat a well layer contains substantially no n-type impurity, a welllayer is grown undoped. In addition, when a barrier layer is doped withn-type impurity, all barrier layers in an active layer may be doped, apart thereof may be doped and a part thereof may be undoped. When a partof barrier layers are doped with n-type impurity, it is preferable thata barrier layer arranged on an n-type layer side in an active layer isdoped. Specifically, by doping an n-th barrier layer B_(n) (n=1, 2, 3 .. . ) counting from an n-type layer side, electrons are effectivelyinjected into an active layer, resulting in a device excellent in theemitting efficacy and the internal quantum efficacy. This applies notonly to a barrier layer but also to a well layer. In addition, when bothare doped, by doping n-th barrier layer B_(n) (n=1, 2, 3 . . . )counting from an n-type layer and a m-th well layer W_(m) (m=1, 2, 3 . .. ), that is, by doping from a side near an n-type layer, there is atendency that the aforementioned effects are obtained.

In addition, as shown Examples to be described later, when a Mg-dopedp-side electron confining layer is provided, in particular, provided incontact with an active layer and/or a barrier layer, since Mg isdiffused, when a p-side barrier layer arranged on a most p-type layerside in an active layer is doped with n-impurity, there is a tendencythat codoping occurs and the function of an active layer isdeteriorated. For this reason, when a Mg-doped p-side electron confininglayer is provided, preferably, this can be avoided by adopting thisp-side barrier layer containing substantially no n-type impurity.Specifically, n-type impurity is less than 5×10¹⁶/cm³.

A thickness of a barrier layer is not particularly limited but is notgreater than 50 nm so as to constitute a quantum well structure.Preferably, a thickness is in a range of not less than 1 nm and notgreater than 30 nm as in a well layer. The reason is as follows: Byadopting not greater than 30 nm, deterioration of the crystallizingproperty is suppressed, by adopting not less than 1 nm, such a thicknessis obtained that a barrier layer functions well. More preferably, athickness is not less than 2 nm and not greater than 20 nm, whereby,when a thickness is not less than 2 nm, a relatively uniform membrane isformed, a barrier layer is provided with the better function and, when athickness is not greater than 20 nm, the better crystallizing propertyis obtained.

An active layer having a quantum well structure in the presentinvention, a preferable embodiment has one or more pairs of a well layercomprising the aforementioned quaternary mixed crystalAl_(x)In_(y)Ga_(1−x−y)N (0<x<1, 0<y<1, x+y<1) and a barrier layercomprising quaternary mixed crystal Al_(u)In_(v)Ga_(1−u−v)N (0<u<1,0<v<1, u+v<1) or ternary mixed crystal Al_(u)Ga_(1−u)N (0<u<1).Specifically, as shown in FIG. 7, an active layer 12 has one or moreInAlGaN well layers 1 and one or more InAlGaN or AlGaN barrier layers 2.Whereby, a well layer of a nitride semiconductor containing In resultsin a well layer excellent in the internal quantum efficacy and theemitting efficacy. Further, by adjusting an Al ratio with a nitridesemiconductor containing Al, as shown in FIG. 10, a well layer can beobtained which can emit at a short wavelength region of 380 nm orshorter. In addition, by forming a barrier layer 12 having the greaterband gap energy than that of a well layer 1 of InAlGaN or AlGaN, also inthe aforementioned short wavelength region, an excellent barrier layercan be obtained.

As seen from FIG. 11, at a region of a wavelength of 370 nm or longer,even when an In crystal mixing ratio x in a well layer is made to be notless than an Al crystal mixing ratio y (x≧y), a threshold currentdensity is not greatly changed and a laser device having the betteroscillating property is obtained. That is, by adopting x≧y in a range ofan Al crystal mixing ratio y of 0<y≦0.1, a better light emitting deviceand laser device can be obtained. On the other hand, as shown in FIG.10, by adopting an Al crystal mixing ratio y in a well layer greaterthan an In crystal mixing ratio x (y≧x), short wavelength light emissionis obtained at a range of an emitting wavelength (oscillatingwavelength) of not greater than 380 nm. That is, by increasing an Alcrystal mixing ratio y as compared with x (y≧x) in a range of an Incrystal mixing ratio x in a well layer of 0<x≦0.1, short wavelengthemitting is obtained. In the relationship between an Al crystal mixingratio and an In crystal mixing ratio in a well layer, by adopting a Gacrystal mixing ratio z (z=1−x−y) greater than an In crystal mixing ratiox and an Al crystal mixing ratio y in the well layer of theaforementioned quaternary mixed crystal InAlGaN, that is, z>x and z>y, awell layer and an active layer showing the aforementioned tendency areobtained. Preferably, quaternary mixed crystal InAlGaN is used so as tobe z>x and z>y in 0<x≦0.1 and 0<y≦0.1.

[Embodiment 1B (Laser Device, Waveguide Structure)]

Another embodiment 1B of the present invention is a laser device having,as a nitride semiconductor device structure, a structure in which anactive layer of the aforementioned first embodiment is held by a firstelectrically conductive type of layer and a second electricallyconductive type of layer. Specifically, as shown in FIG. 2A, the secondembodiment has a structure in which a first electrically conductive typeof layer 11, an active layer 12 and a second electrically conductivetype of layer 13 are laminated on a substrate, and further has astructure in which at least a first light guiding layer 26 is providedin the first electrically conductive type of layer 11 and the secondlight guiding layer 29 is provided in the second electrically conductivetype of layer 13, and an active layer is held by these first and secondlight guiding layers 26 and 29, and in which a waveguide is formed bythe first and second light guiding layers and the active layertherebetween. Further, as described later, when the first electricallyconductive type of layer has a lower cladding layer 25 and the secondelectrically conductive type of layer has an upper cladding layer 30,respectively, a region containing the active layer is held by theseupper and lower cladding layers 25 and 30, resulting in a waveguide.When a light guiding layer is provided in the waveguide held by theupper cladding layer 25 and the lower cladding layer 30, a thresholdcurrent density is decreased, which results in a high output laserdevice. A device structure having a light guiding layer in a waveguidewill be explained below.

In the embodiment 1B of the present invention, as shown in FIG. 2A, awaveguide has a structure in which the active layer 12, the first lightguiding layer 29 in the first electrically conductive type of layer 11and the second light guiding layer 26 in the second electricallyconductive type of layer are provided. The embodiment is a devicecharacterized in a structure in which, in particular, the aforementionedwaveguide using an active layer of a wavelength of 380 nm or shorter isprovided.

This waveguide is for guiding the light mainly from the active layer.Depending upon the structure of this waveguide, the emitting efficacy,the threshold current density and other device properties vary variouslyin a laser device and an end light emitting device. The light guidinglayers are formed on both sides of the active layer like this. The lightguiding layer may be formed in at least one of the first electricallyconductive type of layer and the second electrically conductive type oflayer, that is, either of the first light guiding layer or the secondguiding layer may exist. Preferably, by providing the light guidinglayer on both sides of the active layer, a threshold current density isreduced, resulting in a high output laser device.

As the first light guiding layer 26 or the second light guiding layer 29in the present invention, a nitride semiconductor containing Al is used.In addition, as shown as a band structure 41 in FIGS. 3B to 6, awaveguide structure is obtained by adopting greater band gap energy thanthat of at least a well layer 1 in an active layer 27 in a quantum wellstructure and rendering a difference in a refractive index between theactive layer 27 and the light guiding layers 26 and 29 small. Inaddition, the light guiding layer may have smaller band gap energy thanthat of the barrier layer and, as shown in FIG. 6, or may have greaterband gap energy as shown in FIGS. 3B to 5. As a composition for thelight guiding layer, specifically, In_(α)Al_(β)Ga_(1−α−β)N (0≦α, 0<β,α+β≦1) is used. Preferably, by adopting a nitride semiconductorcontaining no In, that is, by adopting a nitride semiconductor having anIn ratio of zero, the light absorption due to inclusion of In isprevented and a waveguide having suppressed light loss can be obtained.Further, by using preferably Al_(β)Ga_(1−β)N (0≦β≦1), a waveguide isobtained which can be applied to a wide wavelength region fromultraviolet to red. In order to guide the light of a short wavelength ofthe aforementioned 380 nm or shorter, preferably, Al_(β)Ga_(1−β)N(0<β≦1) is used. The reason in as follows: GaN absorbs the light at theaforementioned short wavelength, which results in the loss, and athreshold current density and the current-light output property aredeteriorated. In particular, it is preferable to adjust an Al ratio β inthe light guiding layer so as to be greater than the band gap energyE_(g) in the light guiding layer or the photon energy E_(p) of lightemitting in the active layer by 0.05 eV or greater (E_(g)−E_(p)≧0.05eV). Whereby, a waveguide is obtained in which the light loss due to aguiding layer is suppressed at the aforementioned short wavelength. Morepreferably, by adopting E_(g)−E_(p)≧0.1, a more excellent wavelength isformed.

Here, FIGS. 3A and 3B show a laminated structure 40 and a correspondingband structure 41 in the nitride semiconductor device of the presentinvention. The laminated structure 40 shows the structure in which anactive layer 27 of a quantum well structure having the well layer 1 andthe a difference in a refractive index between the active layer 27 andthe light guiding layers 26 and 29 small. In addition, the light guidinglayer may have smaller band gap energy than that of the barrier layerand, as shown in FIG. 6, or may have greater band gap energy as shown inFIGS. 3B to 5. As a composition for the light guiding layer,specifically, In_(α)Al_(β)Ga_(1−α−β)N (0≦α, 0<β, α+β≦1) is used.Preferably, by adopting a nitride semiconductor containing no In, thatis, by adopting a nitride semiconductor having an In ratio of zero, thelight absorption due to inclusion of In is prevented and a waveguidehaving suppressed light loss can be obtained. Further, by usingpreferably Al_(β)Ga_(1−β)N (0≦β≦1), a waveguide is obtained which can beapplied to a wide wavelength region from ultraviolet to red. In order toguide the light of a short wavelength of the aforementioned 380 nm orshorter, preferably, Al_(β)Ga_(1−β)N (0<β≦1) is used. The reason in asfollows: GaN absorbs the light at the aforementioned short wavelength,which results in the loss, and a threshold current density and thecurrent-light output property are deteriorated. In particular, it ispreferable to adjust an Al ratio β in the light guiding layer so as tobe greater than the band gap energy E_(g) in the light guiding layer orthe photon energy E_(p) of light emitting in the active layer by 0.05 eVor greater (E_(g)−E_(p)≧0.05 eV). Whereby, a waveguide is obtained inwhich the light loss due to a guiding layer is suppressed at theaforementioned short wavelength. More preferably, by adoptingE_(g)−E_(p)≧0.1, a more excellent wavelength is formed.

Here, FIGS. 3A and 3B show a laminated structure 40 and a correspondingband structure 41 in the nitride semiconductor device of the presentinvention. The laminated structure 40 shows the structure in which anactive layer 27 of a quantum well structure having the well layer 1 andthe barrier layer 2 is held by the first electrically conductive type oflayer 11 and the second electrically conductive type of layer 13. FIGS.4 to 6 show the band structure 41 like FIG. 3B

Any one of or both of the first light guiding layer 26 and the secondlight guiding layer 29 may be formed of a single membrane, or may beformed of a multi-layered membrane. When a light guiding layercomprising a single membrane nitride semiconductor is formed, as shownin FIG. 3A, the laminated structure 40 is provided in which the firstlight guiding layer 26 and the second light guiding layer 29 hold theactive layer 27, and its band structure 41 is such that the band gapenergy is greater than that of the active layer. Specifically, theaforementioned Al_(β)Ga_(1−β)N (0≦β≦1) is used and, at theaforementioned short wavelength region, Al_(β)Ga_(1−β)N (0≦β≦1) is used.More preferably, as described above, an Al ratio β is adjusted so thatthe band gap energy E_(g) of the first light guiding layer and that ofthe second light guiding layer are greater than the photon energy E_(p)by 0.05 eV or greater (E_(g)−E_(p)≧0.05 eE, preferably E_(g)−E_(p)≧0.1).

A thickness of the first light guiding layer or the second light guidinglayer is not particularly limited but, specifically, is in a range ofnot less than 10 nm and not greater than 5 μm, preferably in a range ofnot less than 20 nm and not greater than 1 μm, more preferably in arange of not less than 50 nm and not greater than 300 nm. Whereby, thereis a tendency that a waveguide which functions as a guiding layer at notless than 10 nm and which decreases a threshold current density at notless than 20 nm, is formed. There is a tendency that a threshold currentdensity is further decreased at not less than 50 nm. In addition, thereis a tendency that a waveguide functions as a guiding layer at notgreater than 5 μm, decreases the loss during light guiding at notgreater than 1 μm, and further suppresses the light loss at not greaterthan 300 nm.

The light guiding layer in the present invention may be composed of amulti-layered nitride semiconductor. Also in such the case, as describedabove, it is preferable to use a nitride semiconductor containing no In,it is more preferable to use the aforementioned Al_(β)Ga_(1−β)N (0≦β≦1)and, at the aforementioned short wavelength region, it is preferable touse Al_(⊖)Ga_(1−β)N (0<β≦1). This nitride semiconductor is used toobtain a multi-layered membrane in which one or more nitridesemiconductor layers having a different composition are used in eachlight guiding layer. Specifically, a first layer and a second layerhaving a different composition from that of the first layer are used inthe first light guiding layer 26, and a third layer and a fourth layerhaving a different composition from that of the third layer are used inthe second light guiding layer 29. Here, the first through fourth layerscomprise a nitride semiconductor. Whereby, by adopting a different Alratio between the first layer and the second layer, between the thirdlayer and the fourth layer in each guiding layer, a multi-layeredstructure having different band gap energy and refractive index may beobtained.

For example, in a structure in which a first electrically conductivetype of layer, an active layer and a second electrically conductive typeof layer are laminated, such a structure is obtained that a first lightguiding layer has a first layer and a second layer, a second lightguiding layer has a third layer and a fourth layer, the second layer andthe third layer are arranged on the active layer side, and the firstlayer and the fourth layer are arranged on a position far from theactive layer, resulting in a structure in which the band gap energy isstepwisely decreased as approaching the active layer. Specifically, byadopting Al ratios β2 and β2 of the second layer and the third layer onthe active layer side smaller than Al ratios β1 and β4 of the firstlayer and the fourth layer far from the active layer, that is, β1>β2,β4>β3, a step-wise band structure is obtained, carriers are effectivelyinjected into an active layer in a waveguide, and a refractive index ofan active layer and in the vicinity of the active layer becomes larger,whereby, a structure is obtained in which much light is distributed nearthe active layer in the waveguide. Like this, when a light guiding layeris formed of a multi-layered membrane, there is a tendency that increasein an Al ratio deteriorates the crystallizing property. When it isdifficult to form a light guiding layer of a single membrane due todeterioration of the crystallizing property, or when deterioration ofthe property occurs, formation of a multi-layered membrane can suppressdeterioration of the crystallizing property low. In addition, althoughit is possible to increase the band gap energy of the guiding layer(second layer, third layer) near the active layer and decrease arefractive index of the guiding layer, and decrease the band gap energyof the remote guiding layer (first layer, fourth layer) and increase arefractive index of the remote guiding layer by adopting β1<β2, β4<β3contrary to the aforementioned β1>β2, β4>β3, preferably β1>β2, β4>β3 ispreferably adopted because the aforementioned carrier confinement andlight distribution become better. In addition, when a light guidinglayer is formed of a multi-layered membrane, each light guiding layermay be composed of 3 or more layers in addition to aforementioned firstthrough fourth layers, and a guiding layer may be constructed bylaminating a plurality of a first layer (third layer) and a second layer(fourth layer) alternately, that is, by laminating a plurality of pairsof a first layer and a second layer. In addition, in order to form alight guiding layer of a multi-layered membrane, when the aforementionedconditional equation: E_(g)−E_(p)≧0.05 eV is calculated, calculation ismade using an average composition of a whole light guiding layer. Forexample, in the case where a first light guiding layer is constructed ofa first layer comprising Al_(β1)Ga_(1−β1)N (0<β1≦1) having a thicknessof d₁ and a second layer comprising Al_(β2)Ga_(1−β2)N (0<β2≦1, β1≠β2)having a thickness of d₂, an average ratio β_(m) of Al is obtained fromβ_(m)=(d₁×β1+d₂×β2)/(d₁+d₂)

In addition, in a light guiding layer in the present invention, as shownin FIG. 4, a GRIN structure may be adopted in which there is a gradientcomposition so that the band gap energy grows smaller as approaching anactive layer. Specifically, by adopting a gradient Al ratio β, that is,by adopting a gradient composition so that an Al ratio β grows smalleras approaching an active layer, a GRIN structure is obtained, whichresults in improvement of the effects of injecting carriers. Upon this,as shown in FIG. 4, a gradient composition may be continuously gradientcomposition or a discontinuously and stepwisely gradient composition. Inaddition, also in a structure in which a plurality of pairs of firstlayer/second layer for the aforementioned first light guiding layer arelaminated, such as a superlattice multi-layered structure, the band gapenergy may be made to be smaller as approaching an active layer by agradient Al ratio. In this case, only at least one of layers, forexample, only a first layer may have a gradient composition.Alternatively, all layers constituting a pair, for example, a firstlayer and a second layer may have a gradient composition. Alternatively,in a thickness direction of a light guiding layer, there may bepartially a gradient composition, preferably, when all regions in athickness direction have a gradient composition, there is a tendencythat effects of injecting carriers are more improved.

Further, a multi-layered light guiding layer may have a multi-layeredsuperlattice structure as shown FIG. 5. By using a superlatticestructure, deterioration of the crystallizing property due to theaforementioned nitride semiconductor containing Al can be suppressed,and a waveguide having the better crystallizing property can be formed.Specifically, a structure is adopted in which, in the first lightguiding layer 26, the aforementioned first layer and second layer arelaminated so that at least one of them has 2 or more layers, preferably,each layer has 2 or more layers, or a plurality of pairs of the firstlayer and the second layer are laminated. Upon this, a composition of anitride semiconductor in each layer is the same as that described above.Preferably, by using as first layer/second layer Al_(β1)Ga_(1−β1)N(0≦β1≦1)/Al_(β2)Ga_(1−β2)N (0≦β2≦1, β1≠β2) and, at the aforementionedshort wavelength region, Al_(β1)Ga_(1−β1)N (0<β1≦1)/Al_(β2)Ga_(1−β2)N(0<β2≦1, β1≠β2), a waveguide is formed which suppresses the light lossand suppresses deterioration of the crystallizing property by asuperlattice structure. In order that a light guiding layer becomes asuperlattice structure, a thickness of each layer constituting amulti-layered membrane is set so as to form a superlattice. A thicknessis different depending upon a composition and a combination of eachlayer but, specifically, is not greater than 10 nm, preferably notgreater than 7.5 nm, whereby, the crystallizing property can bemaintained better. More preferably, by adopting not greater than 5 nm,the further better crystallizing property can be obtained.

In addition, it is preferable that a light guiding layer in the presentinvention is at least doped with each electrically conductive impuritybecause movement and injection of carriers become better. Upon this,electrically conductive-type impurity may be doped into a part of alight guiding layer or may be partially doped, or may be doped into awhole light guiding layer. In addition, in a multi-layered light guidinglayer, for example, in the first light guiding layer having theaforementioned first layer and second layer, both layers may be doped,or the first layer and the second layer may be doped at a differentamount, or one of them may be doped and the other may be undoped in amodified doping manner. For example, in a superlattice multi-layeredstructure such as a structure in which the first layer and the secondlayer are laminated alternately, or a plurality of pairs are provided inthe aforementioned first light guiding layer, preferably, by doping onlyone of layers, for example, only the first layer in a modified dopingmanner, deterioration of the crystallizing property due to impuritydoping can be suppressed. More preferably, by doping only a layer havinga low Al ratio, a layer having the better crystallizing property can beobtained, deterioration of the crystallizing property due to impuritydoping can be suppressed, and activation due to impurity doping becomesbetter, being preferable. For example, by doping a second layer having asmall Al ratio with impurity and rendering a first layer undoped in afirst light guiding layer having a superlattice multi-layered structurein which the aforementioned first layer/second layer isAl_(β1)Ga_(1−β1)N (0≦β1≦1)/Al_(β2)Ga_(1−β2)N (0<β2≦1, β1<β2), a secondlayer having a smaller Al ratio has the better crystallizing propertythan that of a first layer. For this reason, by doping a layer havingthe better crystallizing property with impurity, the better activationis realized, resulting in a light guiding layer excellent in movementand injection of carriers.

Further, as shown by a change in a doping amount 42 in FIGS. 6A to 6Dregarding impurity doping of the light guiding layer in the presentinvention, by rendering an impurity doping amount smaller as approachingthe active layer, or rendering a doping amount of a region near theactive layer smaller as compared with a region far from the active layerin the first and second light guiding layers 26 and 29, the light lossis further reduced in the waveguide, in particular, in the light guidinglayer, and better light guiding is realized, resulting in decrease in athreshold current density and decrease in driving current. The reason isas follows: When the light is guided in a region doped with impurity,the light absorption occurs due to impurity and the light loss iscaused. Besides, as described above, the waveguide has at least astructure in which the active layer 27 is held by the first lightguiding layer 26 and the second light guiding layer 29, and a structurein which an outside of the guiding layer or the waveguide is held by theupper and lower cladding layers 25 and 30 having a smaller refractiveindex than that of the guiding layer leads to a structure in which thelight is confined in the waveguide, whereby, much light is distributedin the active layer in the waveguide or in the vicinity of the activelayer. For that reason, by rendering an impurity doping amount smallerin the region in the vicinity of the active layer, the light loss in theregion in which much light is distributed is decreased, resulting in thewaveguide in which the light loss is small. Specifically, when the firstlight guiding layer and the second light guiding layer is divided at aregion at half a thickness of each layer and a region near the activelayer and a region far from the active layer are considered, theelectrically conductive impurity concentration in the region near theactive layer is rendered smaller than the impurity concentration in theregion far from the active layer. The impurity concentration in thelight guiding layer is not particularly limited but, specifically, isnot greater than 5×10¹⁷/cm³ in the region near the active layer. Here,the aforementioned impurity doping indicates doping of the first lightguiding layer with the first electrically conductive-type impurity anddoping of the second lighting layer with the second electricallyconductive-type impurity.

Examples of a form of changing a doping amount in the light guidinglayer include, as shown as changes in a doping amount 42 a, 42 b and 42c in FIGS. 6A to 6D, a form of rendering a doping amount smaller mildlyand continuously as approaching an active layer in each light guidinglayer (42 a), a form of rendering a doping layer discontinuously andstepwisely small (42 b), a form of rendering a change in a step-wisedoping amount and partially setting a change in a doping amount in alight guiding layer (42 c), and a combination thereof. Preferably,rendering a region having a distance from an active layer side of notgreater than 50 nm undoped in the light guiding layer, it becomespossible to decrease the light loss. Preferably, by rendering a regionof 100 nm or smaller undoped, it becomes possible to reduce the lightloss well and to decrease a threshold current density and a drivingcurrent. Upon this, when a region of 50 nm or smaller is rendered anundoped region, a thickness of the light guiding layer is not less than50 nm. When a region of 100 nm or smaller is rendered an undoped region,it goes without saying that the thickness is not less than 100 nm. Uponthis, when the aforementioned undoped region is provided in the lightguiding layer, preferably, it is used in a combination with a lightguiding layer having the aforementioned gradient composition structure.The reason is as follows: as shown in FIG. 4, by adopting a bandstructure in which the band gap energy grows smaller as approaching theactive layer. Even when impurity-non-doped region is provided near theactive layer, a light guiding layer is formed which suppresses thecarrier injection efficacy low. Upon this, it is preferable that a lightguiding layer having a gradient composition has the aforementioned GRINstructure. In addition, in the aforementioned multi-layered structure,even in a structure in which a band gap energy grows smaller asapproaching an active layer, it has the effects of forming an undopedregion. Here, in each light guiding layer, even when not doped withimpurity at the growth, that is, even when a light guiding layer isgrown undoped, the impurity is diffused from an adjacent layer in somecases. In such the case, even the aforementioned region grown undoped isdoped with impurity. Specifically, in Mg preferably used as p-typeimpurity, such the diffusion phenomenon is easily caused. As shown inExample 1, even when a p-side light guiding layer is formed undoped, ap-type impurity is doped therein due to diffusion from adjacent electronconfining layer and cladding layer. When impurity doping is effected bydiffusion like this, as described above, the impurity concentration in aregion near an active layer is rendered smaller than that of a remoteregion. It is preferable that such the doping region is provided in atleast one of light guiding layers. More preferably, provision of suchthe doping region in both light guiding layers, a waveguide is obtainedwhich reduces the light loss.

A layer construction, a form of impurity doping, a composition and athickness in the aforementioned light guiding layer may be the same asor different from those for the first light guiding layer and the secondlight guiding layer. For example, the first light guiding layer is asingle membrane and the second light guiding layer is a multi-layeredmembrane so that layer constructions in both light guiding layers aredifferent.

(Cladding Layer)

The aforementioned the embodiments 1A and 1B be a nitride semiconductordevice having a structure in which a first electrically conductive typeof layer, an active layer and a second electrically conductive type oflayer are laminated, a first electrically conductive type of layer has alower cladding layer, and a second electrically conductive type of layerhas an upper cladding layer. Specifically, as shown in FIG. 2A, a devicehas a structure in which a first electrically conductive type of layer11, an active layer 12 and a second electrically conductive type oflayer 13 are laminated on a substrate, and further has a structure inwhich at least a lower cladding layer 25 is provided in the firstelectrically conductive type of layer 11 and an upper cladding layer 30is provided in the second electrically conductive type of layer 13, andthe active layer is sandwiched between these upper and lower claddinglayers 25 and 30. The aforementioned light guiding layer may be providedin a waveguide sandwiched between the upper cladding layer 25 and thelower cladding layer 30. A device structure having a cladding layer willbe explained below.

A composition of the upper and lower cladding layers 25 and 30 are suchthat the band gap energy is greater than that of an active layer asshown as a band structure 41 in FIGS. 3B to 6A. In addition, when theaforementioned laser device and end light emitting device has first andsecond light guiding layers 26 and 29, the band gap energy is almost thesame as or slightly greater than that of a light guiding layer. Whereby,upper and lower cladding layers function to confine carriers or confinethe light and, when a light guiding layer is harbored, function as alayer for confining the light. As a nitride semiconductor used for acladding layer, a nitride semiconductor containing Al is preferablyused, and a nitride semiconductor represented by In_(a)Al_(b)Ga_(1−a−b)N(0≦a, 0<b, a+b≦1) is used. Preferably, by using a nitride semiconductorhaving an In ratio a of zero, there is a tendency that the loss of thelight is easily generated due to absorption in a cladding layer in anitride semiconductor containing In. For this reason, by usingpreferably a nitride semiconductor represented by Al_(b)Ga_(1−b)N(0<b≦1), the light can be confined well and, when a guiding layer is notprovided, carriers can be confined well. In a laser device and an endlight emitting device, in a structure in which a waveguide is held byupper and lower cladding layers, by setting a sufficient difference in arefractive index between a waveguide and a cladding layer, specifically,between an active layer and/or a light guiding layer, such aconstruction is obtained that the light is confined in waveguide and thelight is guided. In order to set such a difference in a refractiveindex, Al_(b)Ga_(1−b)N (0<b≦1) is preferably used and, by satisfying atleast the relationship: β≦b between an Al ratio (average composition) βin a light guiding layer, preferably, by adopting b−β≧0.05, a sufficientdifference in a refractive index is set. Since light confinement by acladding layer also depends upon a thickness of a cladding layer, acomposition of a nitride semiconductor is determined considering also athickness.

A cladding layer in the present invention may be formed of a singlemembrane, or may be formed of multi-layered membrane, or may have amulti-layered superlattice structure like the aforementioned lightguiding layer. When a cladding layer is formed of a single membrane, byforming a single membrane comprising the aforementioned nitridesemiconductor, it is easier to design a light and carrier confiningstructure and a necessary time for growing a cladding layer can beshortened as compared with formation of a multi-layered membrane. On theother hand, it is difficult to grow a nitride semiconductor containingAl such as AlGaN with the better crystallizing property. In particular,a nitride semiconductor is grown at a thickness greater than a constantthickness as in a single membrane, crack is easily caused.

When a cladding layer is formed of a multi-layered membrane, a pluralityof nitride semiconductors having a different composition are laminated,specifically, a plurality of nitride semiconductors having a differentAl ratio are laminated. When formed of a multi-layered membrane likethis, deterioration of the crystallizing property and occurrence ofcrack as in a single membrane can be suppressed. Specifically, as amulti-layered membrane, a first layer and a second layer having adifferent composition therefrom are laminated to provide a plurality oflayers having different refractive index and band gap energy. Forexample, a multi-layered membrane may be obtained which have a structurein which a first layer having an Al ratio b1 and a second layer havingan Al ratio b2 (b1≠b2) are laminated. Upon this, by adopting aconstruction in which an Al ratio is b1<b2 (0≦b1, b2≦1), the first layerhaving a greater Al ratio can make a refractive index and a band gapenergy greater, and the second layer having a smaller Al ratio cansuppress deterioration of the crystallizing property due to formation ofa first layer. Alternatively, plurality of layers having differentcomposition may be further laminated by laminating a first layer and asecond layer, and laminating a third layer having a differentcomposition from that of the second layer. Alternatively, a structuremay be adopted in which a plurality of a first layer and a second layerare laminated alternately, or a structure may be adopted in which aplurality of pairs of at least a first layer and a second layer may beformed. Since deterioration of the crystallizing property of a nitridesemiconductor containing Al is suppressed and a thickness may berendered greater in such a multi-layered structure, it becomes possibleto obtain a thickness important for confining the light.

In a multi-layered structure cladding layer, by adopting a superlatticestructure, a cladding layer can be formed with the much bettercrystallizing property, being preferable. Here, a superlattice structureis provided in at least a part of a cladding layer, preferably, asuperlattice structure is provided in throughout a cladding layer,whereby, a cladding layer may be formed with the better crystallizingproperty. Upon this, a superlattice structure may be a structure inwhich a plurality of at least a first layer and a second layer arelaminated alternately, or plurality of pairs of at least a first layerand a second layer are provided as in a light guiding layer. A thicknessof each layer constituting a superlattice structure is differentdepending upon a composition and a combination of each layer but,specifically, is not greater than 10 nm, preferably not greater than 7.5nm, whereby, the crystallizing property can be maintained better. Morepreferably, by adopting not greater than 5 nm, the much bettercrystallizing property can be obtained.

A cladding layer is preferably doped with at least each electricallyconductive-type impurity, and may be doped totally or partially like alight guiding layer. In addition, also in the case of multi-layeredmembrane, for example, in a multi-layered membrane having the firstlayer and the second layer, both layers may be doped, or a first layerand the second layer may be doped at a different amount, or one oflayers may be doped and the other may be undoped in a modified dopingmanner like the light guiding layer. For example, in the case of asuperlattice multi-layered structure in which the aforementioned firstlayer/second layer is Al_(b1)Ga_(1−b1)N (0≦b1≦1)/Al_(b2)Ga_(1−b2)N(0<b2≦1, b1<b2), by doping the second layer having a smaller Al ratiowith impurity and by rendering the first layer undoped, thecrystallizing property can be made better like the light guiding layer.

A thickness of the cladding layer is not particularly limited but is ina range of not less than 10 nm and not greater than 2 μm, not less than50 nm and not greater than 1 μm. When a thickness is not less than 10nm, carriers can be confined. When a thickness is not greater than 2 μm,deterioration of the crystallizing property can be suppressed. Further,when a thickness is not less than 50 nm, the light can be confined.Whereby, a cladding layer can be used in a laser device and end lightemitting device. When a thickness is not greater than 1 μm, a claddinglayer can be formed with the better crystallizing property.

Here, as an upper cladding layer and a lower cladding layer, a nitridesemiconductor containing Al is preferably used, whereby, a difference ina refractive index can be rendered great between a waveguide and bothcladding layers. Upon this, it is preferable that a nitridesemiconductor for a cladding layer contains no In because a nitridesemiconductor containing In tends to deteriorate the crystallizingproperty as compared with a nitride semiconductor containing no In. Inparticular, in a structure having a p-side cladding layer on an activelayer, when a nitride semiconductor containing In is used for a p-sidecladding layer, the crystallizing property is remarkably deterioratedand the device properties are remarkably deteriorated. Upon this, as anitride semiconductor used in a cladding layer, specificallyAl_(b)Ga_(1−b)N (0<b<1) is preferably used.

(Carrier Confining Layer<p-Side Electron Confining Layer>)

In the present invention, a carrier confining layer 28 may be providedin the interior of the active layer 27 or in the vicinity of the activelayer as shown by a band structure 41 in FIGS. 3 Band 4. As shown by thefigures, in the case of a structure having the light guiding layers 26and 29 and the cladding layers 25 and 30 such as a laser device and anend light emitting device, a carrier confining layer may be providedbetween the light guiding layers 26 and 29 and the active layer 27, oras a part of an active layer or a light guiding layer. Here, thiscarrier confining layer is to confine carriers in an active layer or ina well layer. In a laser device and high output light emitting device,overflow of carriers from an active layer due to a rise in temperatureby device driving and increase in a current density can be prevented,resulting in a structure in which carriers are effectively injected inthe active layer. Specifically, as shown in FIG. 4, carriers from thefirst electrically conductive type of layer are confined by a carrierconfined layer 28 b arranged on the second electrically conductive typeof layer side, and carriers form the second electrically conductive typeof layer are confined by a carrier confining layer 28 a on the firstelectrically conductive type of layer side. It is preferable that thiscarrier confining layer is provided in at least one of them. As shown inExample 1, in the device in which the first electrically conductive typeof layer is n-type and the second electrically layer is p-type, it ispreferable that the carrier confining layer is provided at least on ap-type layer side. The reason is as follows: In a nitride semiconductor,since a diffusion length for an electron is longer as compared with adiffusion length for a hole, an electron is easier to overflow from anactive layer and, for this reason, by providing the carrier confininglayer 28 for confining electrons on the p-type layer side, high outputlaser device and light emitting device are obtained. An example in whicha carrier confining layer as a p-side electron confining layer isprovided on a p-type layer side will be explained below. This can beapplied to an n-type layer side by substituting an electricallyconductive type of layer. In particular, it is preferable that at leasta p-side electron confining layer is provided because an electron aslonger carrier diffusion length as compared with a hole and, therefore,an electron is easier to overflow from an active layer.

As this p-side electron confining layer, a nitride semiconductorcontaining Al, specifically Al_(c)Ga_(1−c)N (0<c<1) is used. Upon this,an Al ratio c is at least in a range of 0.1≦c<1, preferably in a rangeof 0.2≦c<0.5 because a carrier confining layer needs to have asufficiently greater band gap energy than that of the active layer(offset is set). The reason is as follows: When c is not greater than0.1, an electron confining layer does not sufficiently function in alaser device. When c is not less than 0.2, electrons are sufficientlyconfined (carrier confinement), and overflow of carriers are suppressed.In addition, when c is not greater than 0.5, an electron confining layercan be grown while occurrence of crack is suppressed low. Morepreferably, when c is not greater than 0.35, an electron confining layercan be grown with the better crystallizing property. In addition, whenthe aforementioned light guiding layer is harbored, it is preferable toadopt a carrier confining layer having a greater band gap energy thanthat of the light guiding layer. When the aforementioned cladding layeris harbored, it is preferable to adopt a carrier confining layer havinga band gap energy approximately the same as or greater than that of acladding layer. The reason is as follows: For confining carriers, anitride semiconductor having a higher crystal mixing ratio than that ofa cladding layer for confining the light is necessary. This p-sideelectron confining layer can be used in a nitride semiconductor deviceof the present invention. In particular, in the case where a largeamount of carriers are injected in an active layer by driving by a greatcurrent as in a laser device, carriers can be more effectively confinedas compared with no p-side electron confining layer, which can be usednot only in a laser device but also in high output LED.

A thickness of the carrier confining layer in the present invention isat least not greater than 100 nm, preferably not greater than 40 nm. Thereason is as follows: Since a nitride semiconductor containing Al has agreater bulk resistance as compared with other nitride semiconductors(containing no Al) and an Al crystal mixing ratio in a p-side electronconfining layer is set at higher as described above, when a carrierconfining layer is provided at a thickness exceeding 100 nm in a device,it becomes an extremely high resistant layer, leading to a remarkableincrease in forward voltage Vf. When a thickness is greater than 40 nm,a rise in Vf can be suppressed low. More preferably, by adopting notgreater than 20 nm, the rise can be suppressed lower. Here, a lowerlimit from a thickness of a p-side electron confining layer is at leastnot less than 1 nm, preferably not less than 5 nm, whereby, an electronconfining layer functions well. Here, a carrier confining layer may beformed of a single membrane, or may be formed of a multi-layeredmembrane having a different composition.

In addition, in the nitride semiconductor device of the presentinvention, in the case where a light guiding layer is not provided andonly a cladding layer is provided, when sufficient band offset forconfining carriers exists between an active layer and a cladding layeras described above, it is not necessary to provide a carrier confininglayer separately from a cladding layer. However, in the case where acladding layer is arranged apart from an active layer as a structurehaving a light guiding layer, it is better to provide a carrierconfining layer between an active layer and a cladding layer, preferablyin the vicinity of an active layer. The reason is as follows: When acarrier confining layer is provided at a position apart from an activelayer, the effects of suppressing the aforementioned overflow ofcarriers are lost. Specifically, by adopting a distance between anactive layer and a p-side electron confining layer (carrier confininglayer) of not greater than 100 nm, a carrier confining layer functions.More preferably, by adopting the distance of not greater than 500 Å,better carrier confinement becomes possible. When a carrier confininglayer is arranged outside an active layer, most preferably, by arrangingin contact with an active layer, carriers are most effectively confinedin an active layer. When arranged in the interior of an active layer, itcan be provided as a barrier layer or as a part of thereof.Specifically, by arranging at a position nearest each electricallyconductive type of layer in an active layer, that is, as an outermostlayer in an active layer, carriers are effectively injected in a welllayer in the interior of an active layer. For example, in FIG. 4, byproviding the carrier confining layer 28 as an outermost barrier layerin an active layer, it becomes a layer nearest each electricallyconductive type of layer. In the case where a carrier confining layer isprovided in an active layer like this, a band gap energy is renderedgreater as compared with a barrier layer in the interior of an activelayer. A barrier layer in the interior or an active layer is a barrierlayer except for an outermost side and is a barrier layer held by welllayers.

A p-side electron confining layer (carrier confining layer) in thepresent invention may be undoped or may be doped with p-type impurity(each electrically conductive-type impurity). Preferably, a layer isdoped with each electrically conductive-type impurity. For example, bydoping a p-side electron confining layer with p-type impurity, amobility of a carrier is enhanced by doping and Vf can be decreased.When driven by a great current such as a laser device and high powerLED, in order to enhance a mobility of a carrier, it is preferable todope at the high concentration. A specific doping amount is at least5×10¹⁶/cm³ or greater, preferably 1×10¹⁸/cm³ or greater. In theaforementioned device driven by a great current, the doping amount is1×10¹⁸/cm³ or greater, preferably 1×10¹⁹/cm³ or greater. An upper limitof an amount of p-type impurity is not particularly limited but is notgreater than 1×10²¹/cm³. When an amount of p-type impurity grows larger,a bulk resistance tends to increase and, as a result, Vf is increased.In order to avoid this, preferably, an upper limit is the minimum p-typeimpurity concentration which can maintain necessary carrier mobility.Alternatively, by forming a carrier confining layer undoped, doping maybe done by diffusion of impurity from an adjacent layer.

In addition, when a p-type carrier confining layer is provided on ann-side, it is not necessary to set great band offset between an activelayer and a barrier layer like the aforementioned p-side electronconfining layer. When voltage is applied to a device, offset forconfining electrons becomes smaller and, therefore, a confining layer ofa nitride semiconductor having a great Al ratio is necessary. However,since offset for confining holes is slightly changed, it is notnecessary to set an Al ratio as high as a p-side electron confininglayer. Specifically, an n-side barrier layer arranged on a most n-sidein an active layer can function as a hole confining layer. Inparticular, by adopting a thickness of not less than 10 nm, theexcellent hole confining function is exerted. That is, as shown inExample, an n-side barrier layer 2 a can function to confine carriers byrendering a thickness larger as compared with other barrier layers. In amultiple quantum well structure, since other barrier layers 2 b and 2 chave a structure in which they are held by well layers, when a thicknessis rendered greater, carriers are prevented from injecting into a welllayer effectively in some cases. On the other hand, since the n-sidebarrier layer 2 a is formed not held by well layers, by enhancing thefunction of carrier confinement, a better structure of an active layeris obtained. Since this n-side barrier layer is preferably a layerarranged on an outermost side in an active layer, the n-side barrierlayer effectively functions to confine carriers. An upper limit of athickness is not particularly limited but is not greater than 30 nm.Alternatively, it may be formed of a multi-layered membrane. Also in asingle quantum well structure, allowing an n-side barrier layer 2 a tofunction to confine carriers, carriers can be suitably injected in awell layer.

In a laser device and an end light emitting device of the nitridesemiconductor of the present invention, as shown in Examples, ridge isprovided as a stripe-like waveguide and an insulating membrane which isto be an embedding layer is formed on a ridge-side. Upon this, anembedding layer is desirably formed of oxides containing at least oneelement selected from the group consisting of Ti, V, Zr, Nb, Hf and Tain addition to SiO₂ as a material for a second protecting membrane, andat least one of SiN, Bn, SiC and AlN. Among them, it is particularlypreferable to use an oxide of Zr and Hf, and Bn and SiC. Further, as anembedding layer, semi-insulating and i-type nitride semiconductor, anelectrically conductive type which is reverse to a ridge part, inExamples, an n-type nitride semiconductor may be used. By setting adifference in a refractive index by a nitride semiconductor containingAl such as AlGaN and allowing to function as a current preventing layer,lateral light confinement is realized. By setting a difference in alight absorbing coefficient by a nitride semiconductor containing In,optical properties of a laser device are realized. In addition, withoutproviding ridge with etching, a structure in which a current is flowncan be obtained by injecting ions of B and Al and making a non-injectingregion stripe-like.

In addition, by adopting a ridge width of not less than 1 μm and notgreater than 3 μm, preferably not less than 1.5 μm and not greater than2 μm, spot-shape or beam-shape laser light excellent as a light sourcefor optical disk system is obtained.

(Embodiment 2)

Embodiment 2 of the present invention will be explained below, and theembodiment 2 may be used by combining with the aforementioned embodiment1A and/or 1B.

(Active Layer)

An active layer in the present invention preferably has a quantum wellstructure, has a well structure comprising a nitride semiconductorcontaining GaN or Al, and has a barrier layer formed of a nitridesemiconductor containing Al or a nitride semiconductor containing In andAl. In addition, in particular, as a wavelength in an active layer, ashort wavelength having light emitting at 375 nm or shorter ispreferably used and, more specifically, the band gap energy of theaforementioned well layer is of a wavelength of 375 nm or shorter. Uponthis, a nitride semiconductor used in an active layer may be eithernon-doped, n-type impurity-doped, or p-type impurity-doped. Preferably,by provision of non-doped or undoped, or n-type impurity-doped nitridesemiconductor, the high output can be realized in a nitridesemiconductor device such as a laser device and a light emitting device.Preferably, by allowing a well layer to be undoped and a barrier layerto be n-type impurity-doped, such an device is obtained that a laserdevice and a light emitting device have the high output and the emittingefficacy is high. Here, a quantum well structure may be a multiplequantum well structure or a single quantum well structure. Preferably, amultiple quantum well structure makes it possible to improve the outputand decrease an oscillating threshold. As a quantum well structure foran active layer, a structure in which at least one aforementioned welllayer and at least one aforementioned barrier layer are laminated, canbe used. Upon this, in the case of a quantum well structure, the numberof well layers of not less than 1 and not greater than 4 enables toreduce a threshold current in a light emitting device, being preferable.More preferably, a multiple quantum well structure having the number ofwell layers tends to give a high output laser device and light emittingdevice.

It is preferable that a nitride semiconductor containing GaN or Al isused as a well layer in the present invention and that at least one welllayer formed of a nitride semiconductor containing the GaN or Al isharbored in an active layer. In a multiple quantum well structure,preferably, by provision of all well layers comprising a well layercomprising the aforementioned nitride semiconductor, a shorterwavelength is permitted and high output light emitting devices and laserdevices can be obtained. When an emitting spectrum has a nearly singlepeak, this construction is preferable and, on the other hand, in amulti-color light emitting device having a plurality of peaks, byprovision of at least one well layer comprising a nitride semiconductorcontaining the GaN or Al, an emitting peak of a shorter wavelength canbe obtained, and light emitting devices of various emitting colors, or alight emitting apparatus combined with a fluorescent substance which isexcited at that shorter wavelength can be obtained. Upon this, in thecase of a multi-color emitting device, by using In_(α)Ga_(1−α)N (0<α≦1)as a specific composition, a well layer which enables better emissionand oscillation at a range of from an ultraviolet to a visible area isobtained. Upon this, an emitting wavelength can be determined by an Incrystal mixing ratio.

A well layer comprising a nitride semiconductor containing Al in thepresent invention can give a wavelength range which is difficult in theprevious InGaN well layer, specifically, a wavelength around 365 nmwhich corresponds to the band gap energy of GaN, or a shorterwavelength. A particular well layer has the band gap energy by whichemission and oscillation is possible at a wavelength of 375 nm orshorter. In the previous InGaN well layer, at a wavelength of around 365nm which corresponds to the band gap of GaN, for example, at 370 nm, itis necessary to adjust an In ratio around 1% or smaller and, when an Inratio becomes extremely small like this, the emitting efficacy isreduced and, thus, an emitting device and a laser device having thesufficient output is obtained with difficulty. In addition, when an Inratio is 1% or smaller, it is difficult to control the growth. In thepresent invention, by using preferably a well layer comprising a nitridesemiconductor containing GaN or Al, an Al ratio is increased to increasethe band gap energy in a wavelength area of 375 nm at which effectiveemission was difficult previously, which can be used in a shorterwavelength laser device.

Here, a specific composition of a nitride semiconductor containing Alused in a well layer is a composition represented byAl_(x)In_(y)Ga_(1−x−y)N (0<x≦1, 0<y≦1, x+y<1), and a preferablecomposition is Al_(x)Ga_(1−x)N (0<x≦1). A composition of a preferablewell layer in the present invention including the case where theaforementioned well layer is GaN is in using a nitride semiconductorrepresented by Al_(x)Ga_(1−x)N (0≦x≦1). In a method of vaporphase-growing such as MOCVD and the like used for growing a nitridesemiconductor, when the number of constituent devices becomes larger, areaction is easily caused between constituent elements. For this reason,although pluralization of quinary or more mixed crystal is possible byusing B, P, As, Sb and the like as described above, preferably, by usingquaternary mixed crystal of AlInGaN, a reaction between these elementsis prevented to grow a crystal with the better crystallizing property.Further, in quaternary mixed crystal of the aforementioned compositionAl_(x)In_(y)Ga_(1−x−y)N, since there is a tendency that a reactionbetween Al and In at growth becomes problematic in deterioration of thecrystallizing property, preferably, by using Al_(x)Ga_(1−x)N, it becomespossible to form a well layer with the further better crystallizingproperty. In these nitride semiconductors containing Al, by increasingan Al crystal mixing ratio, an device can be obtained which can beemitted and oscillated at the aforementioned shorter wavelength area(λ≦375 nm). Here, an Al ratio x is not particularly limited, but awavelength corresponding to the desired band gap energy can be obtainedby varying an Al ratio.

In a preferable embodiment, an active layer for a quantum well structurein the present invention has one or more pairs of a well layercomprising the aforementioned binary or ternary mixed crystalAl_(x)Ga_(1−x)N (0≦x≦1), and a barrier layer comprising a quaternarymixed crystal Al_(u)In_(v)Ga_(1−u−v)N (0<u<1, 0<v<1, u+v<1) or ternarymixed crystal Al_(u)Ga_(1−u)N (0<u<1).

More specifically, as shown as an active layer 12 in FIGS. 12A, 12B andFIGS. 14A, 14B, an active layer has 1 or more AlGaN well layers 1 and 1or more InAlGaN or AlGaN barrier layers 2. Whereby, a well layerexcellent in the internal quantum efficacy and the light emittingefficacy is obtained. Further, by adjusting an Al ratio by a nitridesemiconductor containing Al, as shown in FIG. 12A, a well layer isobtained which can be emitted at a shorter wavelength area of 375 nm orshorter. In addition, by adopting InAlGaN or AlGaN in a barrier layer 2having the greater band gap energy than that of the well layer 1, abarrier layer can be provided which is also excellent in theaforementioned shorter wavelength area.

(Active Layer and Adjacent Layer)

In the embodiment 2 of the present invention, in a structure in which afirst electrically conductive type of layer and a second electricallyconductive type of layer on both sides of an active layer are laminated,in particular, the relationship will be explained in detail belowbetween a layer arranged near an active layer, more specifically, alayer arranged adjacent to and in contact with an active layer, and anactive layer.

The previously proposed laser device has a structure in which the bandgap energy becomes larger in light guiding layers 26 and 29 on bothsides of an active layer, and cladding layers 25 and 30 on both outersides thereof in this order, as the band structure is shown in FIGS. 13Aand 13B and, a change in an Al crystal mixing ratio in a laminatedstructure of FIG. 2A is shown in FIG. 16. For example, in an AlGaN/InGaNseries nitride semiconductor laser device having a wavelength of 410 nm,in FIG. 16, by adopting an Al ratio in light guiding layers 26 and 29 ofzero as an origin and substituting with an In crystal mixing ratio in anactive layer having the smaller band gap energy than them, a band gapstructure of the previous device is obtained. In addition, in theprevious AlGaN series semiconductor laser device at a shorter wavelengthin an ultraviolet area, as shown in FIG. 16, a structure has beenproposed in which an Al crystal mixing ratio is increased in lightguiding layers 26 and 39 outside an active layer, and a further externalcladding layer in this order, whereby, the band gap energy is increasedfrom an active layer toward an outside as shown in FIGS. 13A and 13B. Inaddition, in the previous AlGaN series nitride semiconductor lightemitting device which emits in an ultraviolet area, a structure isproposed in which a cladding layer or a light guiding layer is omittedin the aforementioned laser device. More specifically, a structure hasbeen proposed in which light guiding layers 26 and 29, and claddinglayers 25 and 30 shown in FIG. 16 are used as a carrier confining layer,that is, an Al ratio is greater than that of a light emitting layer(active layer 27) and, thus, a layer having the great band gap energy isformed. However, in a structure in which an Al crystal mixing ratio isincreased successively toward an outside of an active layer, therearises a serious problem from deterioration of the crystallizingproperty, in particular occurrence of crack.

In the present invention, as shown in FIG. 2A, by adopting a structurein which both light guiding layers 26 and 29 holding an active layer 27have the smaller band gap energy than that of a barrier layer 2 in anactive layer and an Al crystal mixing ratio is small, occurrence ofcrack in the aforementioned previous structure can be suitablysuppressed, and a structure can be obtained in which continuousoscillation is possible at room temperature. More specifically, a firstnitride semiconductor layer is provided in a first electricallyconductive type of layer, and the band gap energy of the fist nitridesemiconductor layer is smaller than that of a barrier layer in an activelayer, that is, in an AlGaN series active layer, an Al crystal mixingratio in a first nitride semiconductor layer which is smaller than thatof a barrier layer is made small. Upon this, the relationship between awell layer and a first nitride semiconductor layer is that, in a welllayer in an active layer, the band gap energy of a first nitridesemiconductor layer is made larger than that of a well layer for lightemitting recombination. In addition, this relationship can be alsoapplied to a second electrically conductive type of layer. Morespecifically, the band gap energy of a second nitride semiconductorlayer in a second electrically semiconductor layer is made smaller thanthat of a barrier layer in an active layer, an Al crystal mixing ratioof a second nitride semiconductor layer is made smaller. By using afirst nitride semiconductor layer (second nitride semiconductor layer)having a smaller Al crystal mixing ratio than that of a barrier layerand arranging the nitride semiconductor layer near, preferably, adjacentto an active layer, an active layer having the better carrierconfinement and the better crystallizing property can be realized. And,by using these layers as a light guiding layer, a waveguide structurewhich is suitable in a shorter wavelength is formed. This will beexplained in more detail below.

As shown in FIGS. 2A and 12A, a nitride semiconductor device inaccordance with one embodiment of the present invention has a structurein which an active layer 12 is provided between a first electricallyconductive type of layer 11 and a second electrically conductive type oflayer 13. As a specific laminate structure, as shown in figures, thedevice has a structure in which, as a first electrically conductive typeof layer 11, a contact layer 23, a lower cladding layer 25 and a lowerlight guiding layer 26 are successively laminated and, thereabove, anactive layer 27 and, above an active layer, as a second electricallyconductive type of layer 13, a carrier-containment layer 28, an upperlight guiding layer 29, an upper cladding layer 30 and a contact layer24 are successively laminated. Here, mutually adjacent layers of thecarrier confining layer, the light guiding layer, the cladding layer andthe contact layer are not limited to the case where contacted as shownin figures, but they may be isolated by providing another layer betweenrespective layers.

Here, FIG. 2A is a cross-sectional view showing a laminated structure ofan device having a waveguide structure in the present invention, andFIGS. 12A and 12B show cases where an active layer and a laminatedstructure 40 of layers near the active layer arranged so as to hold theactive layer, and a band structure 41 in a biased status correspondingto the laminated structure 40, in particular a first electricallyconductive type of layer 11 are on a n-type layer side, and a secondelectrically conductive type of layer 13 is on a p-type layer side. Aband structure 41 in FIGS. 13A, 13B and FIGS. 14A and 14B are the sameas that in FIG. 12B. In figures, a white circle denotes a hole, a blackcircle denotes an electron, an arrow schematically denotes movement ofeach carrier, a solid line denotes a conduction band E_(c) and a valanceband E_(v), and a dotted line denotes a pseudo-Fermi level E_(f). Asseen from FIGS. 12A and 12B, a first nitride semiconductor layer 26 anda second nitride semiconductor layer 29 which have the smaller band gapenergy than that of barrier layers 2 a and 2 b holding a well layer 1,are arranged holding an active layer, and they are used as upper andlower light guiding layers. Here, a carrier confining layer 28 isprovided near, preferably adjacent to an active layer, between a secondnitride semiconductor layer 29 and an active layer 27, in a secondelectrically conductive type of layer (P-type layer side). That is,holes are confined in a well layer by a barrier layer 2 a in an activelayer, electrons are confined by a carrier confining layer 28 adjacentto a barrier layer 2 b and/or an active layer 27. In FIGS. 13A and 13Bshowing the conventional structure, offset for confining carriers isprovided between a layer 26 in a first electrically conductive type oflayer, and an active layer 27 and a barrier layer 2 a. A nitridesemiconductor layer or a light guiding layer 26 having the greater bandgap energy than that of an active layer 27 or a barrier layer 2 a isprovided adjacent to an active layer, and functions to confine carriers.However, in a nitride semiconductor layer 26 adjacent to an active layer27 and a barrier layer 2 a, there is no structure in which carriers areconfined in an active layer. Carriers are confined in a well layer 1 aby a first barrier layer 2 a arranged on a most first electricallyconductive type of layer side.

The relationship between a well layer, a barrier layer and a firstnitride semiconductor layer (second nitride semiconductor layer) will beexplained below. As described above, a nitride semiconductor device ofthe present invention has a structure in which a first electricallyconductive type of layer, an active layer and a second electricallyconductive type of layer are laminated. Here, a case will be explainedwhere a first electrically conductive type of layer is a n-type layerhaving a n-type nitride semiconductor and a second electricallyconductive type of layer is a p-type layer having a p-type nitridesemiconductor. As described above, a case will be explained where, in anactive layer for a quantum well structure, a n-side barrier layerarranged nearest a n-type layer side is a first barrier side and, on theother hand, a p-side barrier layer arranged nearest a p-type layer sideis a second barrier layer. Here, in the present invention, in therelationship with a first nitride semiconductor layer provided in afirst electrically conductive type of layer (n-type layer), preferablynear a n-side barrier layer, a first nitride semiconductor layer has thegreater band gap energy than that of a first barrier layer. Therefore,an active layer has at least a first barrier layer and a well layer.Upon this, it is necessary that a first barrier layer is provided on an-type layer side rather than a well layer. For this reason, in thepresent invention, an active layer has at least a well layer, and afirst barrier layer provided on n-type layer side rather than a welllayer. Preferably, provision of a second barrier layer (p-side barrierlayer) provided on a p-type layer side rather than a well layer, such astructure is provided that a well layer is held by at least a firstbarrier layer and a second barrier layer. The reason is as follows:Since a first barrier layer and a second barrier layer which areprovided holding a well layer therebetween are a barrier layer providednearest a n-type layer and nearest p-type layer, respectively, they havethe different functions.

A first barrier layer is a barrier layer arranged nearest n-type layerin an active layer. More preferably, a first barrier layer is providedon an outermost side and nearest a n-type layer in an active layer,further preferably, it is provided in contact with a n-type layer and afirst nitride semiconductor layer. The reason is as follows: Since afirst barrier layer is provided apart from a n-type layer via a welllayer, for example, in the form shown in FIG. 13B, carriers are injectedinto a well layer on a more n-type layer side than a first barrier layer2 a, and carriers are generated overflowing on a n-type layer side and,on the other hand, when overflow on a n-type layer side is inhibited bya thick first barrier layer, carriers are not injected into a well layeron a more n-type layer side, and the function of a well layer such aslight emitting recombination is deteriorated. Conversely, a firstbarrier layer functions as a barrier for confining carriers in a welllayer within an active layer held by a first barrier layer and a p-typelayer, and a second barrier layer functions similarly for confiningcarriers into a well layer between a second barrier layer and a n-typelayer, while a barrier layer held by well layers, for example, barrierlayers 2 c and 2 d in FIGS. 14A and 14B has the function of dispersingand confining carriers into each well layer and, thus, a first barrierlayer and a second barrier layer, and a barrier layer between welllayers have the different functions. For this reason, in order to make agreatest use of the functions of a first barrier layer, it becomespossible to suitably confine carriers into an active layer by arranginga first barrier layer and a second barrier layer on an outermost side inan active layer.

In addition, regarding a second barrier layer (second p-side barrierlayer), instead of provision of this, by provision of a carrierconfining described later outside an active layer, preferably in contactwith an active layer, in a second electrically conductive type of layer(p-type layer), carriers may be confined in a well layer in an activelayer. Preferably, by provision of a second barrier layer in addition tothis carrier confining layer 28 in an active layer, the nature of easydiffusion of an electron as compared with a hole and a tendency of acarrier diffusion length in a nitrogen semiconductor can be improved andthus, a structure can be obtained in which carriers can be confined andinjected suitably in an active layer, particularly in a well layer.Here, like a first barrier layer, a second barrier layer is arranged ona p-type layer (second electrically conductive type of layer) siderather than a well layer, more preferably, arranged nearest a p-typelayer, most preferably, arranged on an outermost side and on a p-typelayer side in an active layer, whereby, carriers can be suitablyinjected. Alternatively, in connection with a carrier confining layer, asecond barrier layer may be arranged apart from a carrier confininglayer. However, by forming a second barrier layer in contact with acarrier confining layer 28 in a p-type layer, supplemental confinementof carriers in a carrier confining layer by a second barrier layer, andinjection into a well layer are possible, being preferable.

n addition, barrier layers other than an outermost barrier layer amongbarrier layers in an active layer, which are arranged nearer a firstelectrically conductive type of layer and a second electricallyconductive type of layer than a well layer in an active layer like theaforementioned first barrier layer and second barrier layer may beprovided such that a barrier layer 2 c is held by a well layer 1 a and awell layer 2 b and a barrier layer 2 d is held by a well layer 1 b and awell layer 1 c, for example, as shown in FIGS. 14A and 14B. Inparticular, in a multiple quantum well structure, by using such thebarrier layer held by well layers, carriers are suitably dispersed,injected and confined in each well layer in a plurality of well layers.That is, they have the different functions from those of theaforementioned first barrier layer 2 a and second barrier layer 2 b.Even when a thickness is smaller than that of a first barrier layer or asecond barrier layer, such the quantum well structure can be obtainedthat the function of a barrier layer held by well layers is notdeteriorated, and a thickness of a whole active layer can be suppressedand increase in Vf can be suppressed, being preferable. In addition, byusing a barrier layer 2 c held by well layers instead of a first barrierlayer 2 a and a second barrier layer 2 b as shown in FIG. 14A, carriersinjected from each electrically conductive type of layer are directlyand suitably confined and injected in an adjacent well layer by thisbarrier layer 2 c having the great barrier intervening well layers,being preferable. Alternatively, by using barrier layers 2 c and 2 dheld by well layers instead of a first barrier layer 2 a and a secondbarrier layer 2 b as shown in FIG. 14B, the function of confinement by abarrier layer situated in the interior of these barrier layers isweakened, and a first barrier layer 2 a and a second barrier layer 2 bsituated in the outside thereof are strengthened as compared with thesebarrier layers. Whereby, even when the number of well layers isincreased, outer barrier layers form the great barrier and, therefore, astructure can be obtained in which injection and confinement of carriersin each well layer are suitably realized.

As explained above, since a first barrier layer 2 a and a second barrierlayer 2 c which are an external barrier layer have the differentfunctions from those of an internal barrier layer held by well layers, athickness, the band gap energy and the composition may be changedbetween an internal barrier layer and an external barrier layer and,thus, an device having the desired device properties can be obtained.Alternatively, in an active layer having a plurality of internal barrierlayers as shown in FIG. 14B, the composition, the band gap energy and athickness may be changed between respective internal barrier layers.Alternatively, the composition, the band gap energy and a thickness maybe approximately the same between respective internal barrier layers.Preferably, by adopting approximately the same composition, band gapenergy and thickness, the approximately uniform function are imparted tointernal barrier layers and, thus, carriers are suitably injected inrespective well layers.

In addition, as described above, for the aforementioned reasons, it ispreferable that, regarding doping of respective barrier layers withimpurity, a first barrier layer 2 b situated on a most n-type layer sideis doped with n-impurity. It is preferable that a second barrier layerarranged on a most p-type layer side is not substantially doped withn-type impurity, more specifically, is doped at the impurityconcentration of 5×10¹⁶/cm³, rather than doped with n-type impurity. Thereason is as follows: N-type impurities used in a nitride semiconductorhave the high diffusibility in many cases. For example, there is atendency that widely used Mg and Zn are extensively diffused in alaminated structure. When p-type impurity is doped in a barrier layer,diffusion into an adjacent well layer occurs, and there is a tendencythat light emitting recombination of carriers in a well layer issuppressed. In addition, by making a second barrier layer near a p-typelayer side undoped, diffusion of impurity from a p-type layer isterminated in a barrier layer, and further diffusion of impurity in awell layer is prevented, being preferable. In particular, when a carrierconfining layer 28 is harbored in a p-type layer and arranged in thevicinity of a second barrier layer, preferably in contact with a secondbarrier layer, since there is a tendency that a carrier confining layerbecomes a relatively high resistant layer, p-type impurity tends to bedoped at the high concentration and, therefore, this impurity diffusionbecomes problematic. However, making a second barrier layer undoped,deterioration of the function of a well layer due to the diffusion canbe prevented, being preferable. In addition since there is a tendencythat a p-n junction is formed in the vicinity of a carrier confininglayer and, as shown in FIGS. 12B and 14A and the like, a carrierconfining layer is formed at the greatest Al crystal mixing ratio in andevice structure, a great piezoelectricity is applied by a nitridesemiconductor having a high Al crystal mixing ratio, which tends to haveadverse effect on a well layer. However, by forming a second barrierlayer having a smaller Al crystal mixing ratio than that of a carrierconfining layer, there is a tendency that adverse effect on a well layercan be suppressed, being preferable.

In addition, when a first barrier layer has a greater thickness thanthat of a second barrier layer in comparison with a first barrier layerand a second barrier layer, by provision of a carrier confining layer 28in a second electrically conductive type of layer, the function ofconfining carriers in an active layer by a second barrier layer isreduced, that is, a barrier layer works like the aforementioned internalbarrier layer. A structure is obtained in which confinement of carriersmainly in an active layer is realized by a carrier confining layer 28.Since a thickness of a whole active layer can be reduced, Vf is reduced.In addition, in a nitride semiconductor, since a diffusion length of ahole is sufficiently smaller than a diffusion length of an electron,when a thickness of a first barrier layer which is an inlet for holes issmall, and carriers are effectively injected in a well layer, beingpreferable. On the other hand, when a carrier confining layer 28 dopedwith p-type impurity is provided, or when a second nitride semiconductorlayer 29 arranged near an active layer, preferably in contact with anactive layer has the greater band gap energy than that of a firstbarrier layer, a layer having a high Al crystal mixing ratio is providedadjacent to an active layer. For this reason, since a layer having ahigh Al crystal mixing ratio is highly resistant, the great heat isproduced in this layer during device operation and, when it is near awell layer, adverse effect of the heat on a well layer occurs, whichtends to decrease the device properties. In addition, at an interfacebetween such the layer having a high Al crystal mixing ratio and anactive layer, or at an interface on an active layer side of a layerhaving a high Al crystal mixing ratio, or in the vicinity thereof, whena p-n junction is formed and a well layer for an active layer isprovided in the vicinity thereof as shown in FIGS. 12B, 14A and 14B,there is a tendency that bias may have adverse effect on light emittingrecombination in a well layer. That is, it is preferable that a firstbarrier layer is made to function as a spacer for isolating a well layerand a layer having a high Al crystal mixing ratio so that theaforementioned layer having a high Al crystal mixing ratio has noadverse effect on a well layer. In this case, when a specific thicknessof a first barrier layer is at least 20 Å or more, the aforementionedfunction as a spacer can be manifested. When a thickness is 40 Å ormore, an active layer which has suppressed influence on a well layer isobtained, being preferable.

As a first light guiding layer 26 and a second light guiding layer 29 inthe present invention, a nitride semiconductor containing Al. Inaddition, as shown as a band structure 41 in FIGS. 12B and FIGS. 14A and14B, light guiding layers are made to have the greater band gap energyat least than that of a well layer 1 in an active layer 27 in a quantumwell structure, and a difference in a refractive index between an activelayer 27 and light guiding layers 26 and 29 is made to be small,whereby, a waveguide structure is obtained. In addition, as shown inFIGS. 12B and FIGS. 14A and 14B, a light guiding layer may have thesmaller band gap energy than that of a barrier layer. Alternatively, asshown in FIGS. 13A and 13B, a part of light guiding layers may have thegreater band gap energy than that of a barrier layer. In this case,light guide layers except for a first barrier layer, or a part thereofmay have the greater band gap energy than that of a barrier layer.Alternatively, as shown in FIGS. 14A and 14B, a light guiding layer mayhave the greater band gap energy than that of an internal barrier layer,that is, a part of barrier layers in an active layer.

That is, preferably, when a light guiding layer has a first nitridesemiconductor layer having the smaller band gap energy than that of afirst barrier layer, more preferably, a light guiding layer comprises afirst nitride semiconductor layer, or a whole light guiding layer hasthe smaller band gap energy than that of a first barrier layer in amulti-layered membrane light guiding layer having a layer other than afirst nitride semiconductor layer, the function as the aforementionedcarrier confining layer for a first barrier layer is suitablymanifested. Further, when a light guiding layer having a low Al crystalmixing ratio is formed, for example, whereby, a lower light guidinglayer is formed, an active layer can be formed while deterioration ofthe crystallizing property due to a nitride semiconductor containing Alis suppressed and, thus, an device excellent in the light emittingdevice and laser device properties can be obtained. Alternatively, inaddition, like provision of a light guiding layer in a firstelectrically conductive type of layer in the case of a first nitridesemiconductor layer, in the case where a light guiding layer is providedin a second electrically conductive type of layer, as described above, asecond nitride semiconductor layer having the smaller band gap energythan that of a second barrier layer may be provided. The same effects asthose of a first nitride semiconductor layer are exerted.

Further, in the case where a second nitride semiconductor layer isprovided in an upper light guiding layer, as the composition of a lightguiding layer, specifically, In_(α)Al_(β)Ga_(1−α−β)N (0≦α, 0<β, α+β≦1)is used. Preferably, by using a nitride semiconductor containing no In,that is, by using nitride semiconductor having an In ratio of zero,absorption of the light due to inclusion of In can be prevented and,thus, a waveguide which can suppress the loss of the light low can beobtained. Further, by using preferably Al_(β)Ga_(1−β)N (0≦β≦1), awaveguide is obtained which can be applied to a wide wavelength areafrom an ultraviolet area to a red area. In particular, in order to guidethe light in a shorter wavelength area of 380 nm or shorter as describedabove, Al_(β)Ga_(1−β)N (0≦β≦1) is preferably used because, in GaN, theaforementioned shorter wavelength area light is absorbed, resulting inthe loss, and the threshold current density and the current-light outputproperties are deteriorated. In particular, it is preferable that an Alratio β in a light guiding layer is adjusted so that the band gap energyE_(g) of a light guiding layer is greater than the photon energy E_(p)of light emitting of an active layer by 0.05 eV or greater(E_(g)−E_(p)≧0.05 eV). Whereby, a waveguide is obtained in which theloss of the light due to a guiding layer is suppressed in theaforementioned shorter wavelength area. More preferably, by adoptingE_(g)−E_(p)≧0.1, a more excellent waveguide is formed.

In case where a first nitride semiconductor layer is provided in a lightguiding layer, the light guiding layer may have a superlattice structureor be formed of a single membrane. The formation of a single membrane,in comparison to the superlattice structure, may facilitate flow ofcarrier current and decrease of Vf. In this case a thickness of thesingle membrane is as thick as at least no quantum effect occurs,preferably thicker than that of a first barrier layer (a second barrierlayer in case of a second nitride semiconductor layer), more preferably300 Å or greater.

In case where a light guiding layer has a superlattics, on the otherhand, all of layers constituting the superlattics may preferably containAl, or it is preferable that at least one of layers constituting thesuperlattics may preferably contain Al and whose bandgap energy issmaller than that of a outermost barrier layer in a active layer andlarger than that of a inner barrier layer in the active layer. This cansufficiently facsilitate carriers to be confined into the active layer.

The interface between the first electrically conductive type of layerand the first barrier layer is preferably lattice-mismatched. Morespecifically, where the first barrier layer is formed ofAl_(u)In_(v)Ga_(1−u−v)N (0<u<1, 0<v<1, u+v<1), the first nitridesemiconductor layer may be formed of Al_(x)Ga_(1−x)N (0≦x<1). Where thefirst electrically conductive type of layer is formed of quaternarymixed crystal of nitride semiconductor to be lattice-matched with thefirst barrier layer, the layer is intended to contain In and thequaternary mixed crystal of nitride semiconductor containing In may bedifficult to form a thickness of, for example, 300 Å or greater. Thusthe first electrically conductive type of layer is preferable formed ofAlGaN excluding In, which has a lattice-mismatching character.

(Embodiment 3)

A third embodiment of the present invention will be explained below,describing specific examples of device structures.

(Embodiment 3-1)

Active layer (AlGaN barrier layer/GaN well layer/AlGaN barrier layer)SCH structure

p-GaN contact layer

p-Al_(0.1)Ga_(0.9)N/Al_(0.05)Ga_(0.95)N superlattice cladding layer(upper cladding layer 30)

p-Al_(0.04)Ga_(0.96)N guiding layer (upper guiding layer 29)

p-Al_(0.3)Gao_(0.7)N (carrier confining layer 28)

Active layer (Al_(0.15)Ga_(0.85)N barrier layer (first barrierlayer)(100 Å)/GaN well layer (100 Å)/Al_(0.15)Ga_(0.85)N barrier layer(second barrier layer)(45 Å))

n-Al_(0.04)Ga_(0.96)N guiding layer (lower guiding layer 26)

n-Al_(0.1)Ga_(0.9)N/Al_(0.05)Ga_(0.95)N superlattice cladding layer(lower cladding lower 25)

n-InGaN crack preventing layer

n-Al_(0.02-0.03)GaN (Si doped: carrier concentration 2×10¹⁸ cm⁻³)

Substrate (defect density 5×10⁵/cm³: substrate obtained bycrystal-growing GaN on an ELOG substrate by a HVPE method)

(Embodiment 3-2)

Active layer (AlInGaN barrier layer/GaN well layer/AlInGaN barrierlayer) SCH structure

An active layer is shown below, and others have the same structure as inEmbodiment 3-1.

Active layer (Al_(0.15)In_(0.03)Ga_(0.82)N barrier layer (first barrierlayer) (100 Å)/GaN well layer (100 Å)/Al_(0.15)In_(0.03)Ga_(0.82)Nbarrier layer (second barrier layer) (45 Å))

(Embodiment 3-3)

Active layer (AlGaN barrier layer/AlGaN well layer/AlGaN barrier layer)SCH structure (oscilating wavelength 360 nm)

An active layer is shown below, and others have the same structure as inEmbodiment 3-1.

Active layer (Al_(0.20)Ga_(0.80)N barrier layer (first barrier layer)(100 Å)/Al_(0.05)Ga_(0.95)N well layer (100 Å)/Al_(0.20)Ga_(0.80)Nbarrier layer (second barrier layer)(45 Å))

(Embodiment 3-4)

Active layer (AlInGaN barrier layer/AlGaN well layer/AlInGaN barrierlayer) SCH structure (oscillating wavelength 360 nm)

An active layer is shown below, and others have the same structure as inEmbodiment 3-1.

Active layer (Al_(0.15)In_(0.03)Ga_(0.82)N barrier layer (first barrierlayer)(100 Å)/GaN well layer (100 Å)/Al_(0.15)In_(0.03)Ga_(0.82)Nbarrier layer (second barrier layer)(45 Å))

(Embodiment 3-5)

Active layer (AlGaN barrier layer/GaN well layer/AlGaN barrier layer)GRIN structure

P-GaN Contact Layer

No light guiding layer is used, a cladding layer is shown below, andothers have the same structure as in Embodiment 3-1.

p-Al_(a)Ga_(1−a)N/Al_(b)Ga_(1−b)N superlattice cladding layer (uppercladding layer 30)

n-Al_(c)Ga_(1−c)N/Al_(d)Ga_(1−d)N superlattice cladding layer (lowercladding layer 25)

(Embodiment 3-6)

Active layer (AlInGaN barrier layer/GaN well layer/AlInGaN barrierlayer) GRIN structure

No light guiding layer is used, a cladding layer and an active layer areshown below, and others have the same structure as in Embodiment 3-1.

p-Al_(a)Ga_(1−a)N/Al_(b)Ga_(1−b)N superlattice cladding layer (uppercladding layer 30)

Active layer (Al_(0.15)In0.03Ga_(0.82)N barrier layer (first barrierlayer) (45 Å)/GaN well layer (100 Å)/Al_(0.15)In_(0.03)Ga_(0.82)Nbarrier layer (second barrier layer)(45 Å))

n-Al_(c)Ga_(1−c)N/Al_(d)Ga_(1−d)N superlattice cladding layer (lowercladding layer 25)

(Embodiment 3-7)

Active layer (AlGaN barrier layer/AlGaN well layer/AlGaN barrier layer)GRIN structure (oscillating wavelength 360 nm)

No light guiding layer is used, a cladding layer and an active layer areshown below, and others have the same structure as in Embodiment 3-1.

p-Al_(a)Ga_(1−a)M/Al_(b)Ga_(1−b)N superlattice cladding layer (uppercladding layer 30)

Active layer (Al_(0.20)Ga_(0.80)N barrier layer (first barrierlayer)(100 Å)/Al_(0.05)Ga_(0.95)N well layer (100 Å)/Al_(0.20)Ga_(0.80)Nbarrier layer (second barrier layer)(45 Å))

n-Al_(c)Ga_(1−c)N/Al_(d)Ga_(1−d)N superlattice cladding layer (lowercladding layer 25)

(Embodiment 3-8)

Active layer (AlInGaN barrier layer/AlGaN well layer/AlInGaN barrierlayer) GRIN structure (oscillating wavelength 360 nm)

No light guiding layer is used, a cladding layer and an active layer areshown below, and others have the same structure as in Embodiment 3-1.

p-Al_(e)Ga_(1-e)N gradient ratio cladding layer (upper cladding layer30)

Active layer (Al_(0.15)In_(0.03)Ga_(0.82)N barrier layer (first barrierlayer)(100 Å)/GaN well layer (100 Å)/Al_(0.15)In_(0.03)Ga_(0.82)Nbarrier layer (second barrier layer)(45 Å))

n-Al_(f)Ga_(1-f)N gradient ratio cladding layer (lower cladding layer25)

(Embodiment 3-9)

Active layer (AlGaN barrier layer/GaN well layer/AlGaN barrier layer)GRIN-SCH structure

A light guiding layer is shown below, and others have the same structureas in Embodiment 3-1.

p-Al_(g)Ga_(1-g)N gradient ratio guiding layer (upper guiding layer 29)

Active layer (Al_(0.15)Ga_(0.85)N barrier layer (first barrierlayer)(100 Å)/GaN well layer (100 Å)/Al_(0.15)Ga_(0.85)N barrier layer(second barrier layer)(45 Å))

n-Al_(h)Ga_(i-h)N gradient ratio guiding layer (lower guiding layer 26)

(Embodiment 3-10)

Active layer (AlInGaN barrier layer/GaN well layer/AlInGaN barrierlayer) GRIN-SCH structure (oscillating wavelength 360 nm)

A cladding layer and an active layer are shown below, and others havethe same structure as in Embodiment 3-1.

p-Al_(g)Ga_(1-g)N gradient ratio guiding layer (upper guiding layer 29)

Active layer (Al_(0.15)In_(0.03)Ga_(0.82)N barrier layer (first barrierlayer)(100 Å)/GaN well layer (100 Å)/Al_(0.15)In_(0.03)Ga_(0.82)Nbarrier layer (second barrier layer)(45 Å))

n-Al_(h)Ga_(i-h)N gradient ratio guiding layer (lower guiding layer 26)

(Embodiment 3-11)

Active layer (AlGaN barrier layer/AlGaN well layer/AlGaN barrier layer)GRIN-SCH structure (oscillating wavelength 360 nm)

A cladding layer and an active layer are shown below, and others havethe same structure as in Embodiment 3-1.

p-Al_(g)Ga_(1-g)N gradient ratio guiding layer (upper guiding layer 29)

Active layer (Al_(0.20)Ga_(0.80)N barrier layer (first barrierlayer)(100 Å)/Al_(0.05)Ga_(0.95)N well layer (100 Å)/Al_(0.20)Ga_(0.80)Nbarrier layer (second barrier layer)(45 Å))

n-Al_(h)Ga_(i)—_(h)N gradient ratio guiding layer (lower guiding layer26)

(Embodiment 3-12)

Active layer (AlInGaN barrier layer/AlGaN well layer/AlInGaN barrierlayer) GRIN-SCH structure (oscillating wavelength 360 nm)

A cladding layer and an active layer are shown below, and others havethe same structure as in Embodiment 3-1.

p-Al_(g)Ga_(1-g)N gradient ratio guiding layer (upper guiding layer 29)

Active layer (Al_(0.15)In_(0.03)Ga_(0.82)N barrier layer (first barrierlayer)(100 Å)/GaN well layer (100 Å)/Al_(0.15)In_(0.03)Ga_(0.82)Nbarrier layer (second barrier layer) (45 Å))

p-Al_(g)Ga_(1-g)N gradient ratio guiding layer (lower guiding layer 26)

The characteristics of the aforementioned Embodiments 3-1˜3-12 will beexplained, respectively.

Embodiment 3-1 has a superlattice structure in which an active layer isheld by upper and lower cladding layers, an upper light guiding layerand a lower light guiding layer are provided between respective claddinglayers and an active layer, one of cladding layers ismodification-doped, respective light guiding layers have the smallerband gap energy than that of a first barrier layer or a second barrierlayer and have a smaller Al crystal mixing ratio, and a barrier layer isformed of an AlGaN ternary mixed crystal. A different point inembodiment 3-2 from embodiment 3-1 is that a first barrier layer and asecond barrier layer are formed of an AlInGaN quaternary mixed crystal.A different point in embodiment 3-3 from embodiments 3-1 and 3-2 is thata well layer is an AlGaN ternary mixed crystal. A different point inembodiment 3-4 from embodiments 3-1, 2 and 3 is that a well layer is anAlGaN ternary mixed crystal and a barrier layer is an AlInGaN quaternarymixed crystal. A different point in embodiment 3-5 from embodiments 3-1,2, 3 and 4 is that one of an upper cladding layer and a lower claddinglayer is doped in a modified doping manner to form a superlatticecladding layer, Al ratios A and C in one of cladding layers are renderedsmaller as approaching an active layer, the band gap energy E_(c) in thevicinity of an active layer (area having a distance from an active layerof 0.1 μm or smaller) is greater than E_(p) by 0.05 Ev or greater and,on the other hand, in this vicinity, an Al ratio and the band gap energyare rendered smaller than those of a first barrier layer and a secondbarrier layer. Upon this, an Al ratio is a>b, c>d. A different point inembodiment 3-8 from embodiments 3-1˜7 is that, in an AlGaN layer as anupper cladding layer and a lower cladding layer, Al crystal mixingratios e and f are rendered smaller as approaching an active layer, theband gap energy E_(c) in the vicinity of an active layer (area having adistance from an active layer of 0.1 μm or smaller) is greater thanE_(p) by 0.05 eV or greater and, on the other hand, in this vicinity, anAl crystal mixing ratio and the band gap energy are rendered smallerthan those of a first barrier layer and a second barrier layer. Adifferent point in embodiment 3-9 from embodiments 3-1˜8 is in that aguiding layer has a gradient ratio structure, Al crystal mixing ratios gand h are rendered smaller as approaching an active layer, a part of theguiding layer has smaller Al crystal mixing ratio and band gap energythan those of a first barrier layer and a second barrier layer.

EXAMPLE 1

As Example, a laser device in which a nitride semiconductor is used in alaser device structure as shown in FIG. 1, and in a waveguide structureas shown in FIG. 1, will be explained below. Here, a first electricallyconductive type of layer is formed of an n-type nitride semiconductorand a second electrically conductive type layer is formed of a p-typenitride semiconductor. However, the present invention is not limited tothis, but conversely a first electrically conductive type of layer maybe p-type and a second electrically conductive type of layer may ben-type.

A GaN substrate is used in this Example. Alternatively, a differentheterogeneous substrate from a nitride semiconductor may be used as asubstrate. As a heterogeneous substrate, substrate materials differentfrom a nitride semiconductor, which have been known that a nitridesemiconductor can be grown thereon, can be used, such as sapphire,spinel (insulating substrate such as MgAl₂O₄), SiC (including 6H, 4H and3C), ZnS, ZnO, GaAs and Si, and oxide substrates which arelattice-compatible with a nitride semiconductor, having a main plane ofeither of C-plane, R-plane and A-plane. Preferable heterogeneoussubstrates are sapphire and spinel. In addition, a heterogeneoussubstrate may be off angle. In this case, when a heterogeneous substratewhich is step-like off angle is used, a ground layer comprising galliumnitride is grown with the better crystallizing property, beingpreferable. Further, when a heterogeneous substrate is used, a devicestructure as a nitride semiconductor single substrate may be formed bygrowing a nitride semiconductor which is to be a ground layer beforeformation of a device structure, and removing a heterogeneous substrateby a method such as abrasion. Alternatively, after formation of a devicestructure, a heterogeneous substrate may be removed. In addition to aGaN substrate, a substrate of a nitride semiconductor such as AlN andthe like may be used.

In the case where a heterogeneous substrate is used, when a devicestructure is formed via a buffering layer (low temperature growinglayer) and a ground layer comprising a nitride semiconductor (preferablyGaN), a nitride semiconductor is grown better. In addition, when anitride semiconductor grown by ELOG (Epitaxially Laterally Overgrowth)is used as a ground layer (growing substrate) provided on aheterogeneous substrate, a growing substrate having the bettercrystallizing property is obtained. As an example of ELOG layer, a maskregion formed by growing a nitride semiconductor layer on aheterogeneous substrate and providing a protecting membrane on which anitride semiconductor is difficult to be grown, and a non-mask region onwhich a nitride semiconductor is to be grown are provided stripe-like, anitride semiconductor is grown from the non-mask region. Whereby,lateral direction growth is effected in addition to a thicknessdirection growth. Whereby, a nitride semiconductor is also grown on amask region to obtain an example of ELOG layer. In other form, anopening is provided in a nitride semiconductor layer grown on aheterogeneous substrate, and lateral direction growth is effected from aside of the opening to obtain an example of ELOG layer.

(Substrate 101)

After a nitride semiconductor, GaN in this Example, grown on aheterogeneous substrate as a substrate is grown at a thickness of 100μm, a heterogeneous substrate is removed, and a nitride semiconductorsubstrate comprising GaN of 80 μm is used. The details of a method forforming a substrate is as follows: a heterogeneous substrate comprising2 inch φ sapphire having C plane as a main plane is placed in a MOVPEreactor, a temperature is maintained at 500° C., trimethylgallium (TMG)and ammonia (NH₃) are used to grow a low temperature growing bufferinglayer comprising GaN at a thickness of 200 Å and, thereafter, atemperature is risen, undoped GaN is grown at a thickness of 1.5 μm toobtain a ground layer. Then, a plurality of stripe-like masks are formedon the surface of a ground layer, a nitride semiconductor, GaN isselectively grown from a mask opening (window part) to form a nitridesemiconductor layer (lateral growing layer) obtained by growth withlateral growing (ELOG), subsequently, GaN is grown at a thickness of 100μm by HVPE, and a heterogeneous substrate, a buffering layer and aground layer are removed to obtain a nitride semiconductor substratecomprising GaN.

Upon this, a mask at selective growth comprises SiO₂. A mask width of 15μm and an opening (window part) width of 5 μm can reduce penetrationrearrangement. Specifically, in a region in which laterally grown suchas an upper part of a mask, penetration rearrangement is reduced. At amask opening, a membrane is obtained by approximate membrane growth.Therefore, there is no change in penetration rearrangement, resulting ina layer in which a region having a great penetration rearrangementdensity and a region having a small density are distributed. For forminga thick nitride semiconductor layer, a HVPE method is preferable owingto great growing rate. By using GaN or AlN as a nitride semiconductor tobe grown by HVPE, a thick membrane is grown with the bettercrystallizing property. When the GaN substrate is formed by HVPE, thereis a tendency of three dimensional growing form in which as a domaingrown from a produced nucleus is grown in a thickness direction,respective domains are combined to form a membrane. In such the case,since penetration rearrangement is transmitted with nucleus growth,there is a tendency that penetration rearrangement distributed by theaforementioned lateral growing layer is dispersed.

(Buffering Layer 102)

At a temperature of 1050° C., TMG (trimethylgallium), TMA(trimethylaluminium) and ammonia are used to grow a buffering layer 102comprising Al_(0.05)Ga_(0.95)N at a thickness of 4 μm on a nitridesemiconductor substrate. This layer functions as a buffering layerbetween an AlGaN n-side contact layer and a nitride semiconductorsubstrate comprising GaN.

Specifically, when a lateral growing layer or a substrate formed byusing the layer is GaN, a buffering layer 102 comprising a nitridesemiconductor having a smaller thermal expansion coefficient than thatof GaN, Al_(a)Ga_(1−a)N (0<a≦1) can be used to reduce pit. Preferably,it is provided on GaN which is a lateral growing layer of a nitridesemiconductor. Further, when an Al crystal mixing ratio a in thebuffering layer 102 is 0<a<0.3, a buffering layer can be formed with thebetter crystallizing property. This buffering layer may be formed as ann-side contact layer. After the buffering layer 102 is formed, an n-sidecontact layer represented by a composition equation of theaforementioned buffering layer is formed, thus, the buffering layer 102and an n-side contact layer 104 thereon allow to have the bufferingeffect. That is, this buffering layer 102 is provided between a nitridesemiconductor substrate using lateral growth or a lateral growing layerformed thereon and a device structure, or between an active layer in adevice structure and a lateral growing layer (substrate), or a lateralgrowing layer formed therein (substrate), more preferably, is providedbetween a substrate-side lower cladding layer in a device structure anda lateral growing layer (substrate) at least one layer, whereby, pit canbe reduced and the device properties can be improved.

In addition, when an n-side contact layer is a buffering layer, in orderto obtain better ohmic contact with an electrode, it is preferable thatan Al crystal mixing ratio a in an n-side contact layer is 0.1 orsmaller. This first nitride semiconductor layer, or a buffering layer tobe provided on a lateral growing layer formed thereon, may be grown at alow temperature of not lower than 300° C. and not higher than 900° C.,at a temperature of not lower than 800° C. and not higher than 1200° C.,like a buffering layer to be provided on the aforementionedheterogeneous substrate. Preferably, when single crystal is grown at atemperature of not lower than 800° C. and not higher than 1200° C.,there is a tendency that the aforementioned pit reducing effects can beobtained. This buffering layer may be doped with n-type or p-typeimpurity, or may be undoped. In order to obtain the better crystallizingproperty, it is preferable that this buffering layer is formed undoped.When 2 or more buffering layers are provided, the n-type or p-typeimpurity concentration, and an Al crystal mixing ratio can be changed.

Then, respective layers which are to be a device structure are laminatedon a ground layer comprising a nitride semiconductor. Here, as a firstelectrically conductive type of layer, n-side contact layer 110 ton-side light guiding layer are provided and, as a second electricallyconductive type of layer, p-side electron confining layer 108 to p-sidecontact layer 111 are provided.

(N-Side Contact Layer 103)

Then, on the resulting buffering layer 102, TMG, TMA, ammonia, andsilane gas as impurity gas are used to grow an n-side contact layer 103comprising Si-doped Al_(0.05)Ga_(0.95)N at a thickness of 4 μm at 1050°C. By using a nitride semiconductor containing Al, specifically,Al_(x)Ga_(1−x)N (0<x≦1) in an n-side contact layer or a ground layersuch as a buffering layer, there is a tendency that deterioration of thecrystallizing property due to use of ELOG, in particular, occurrence ofpit are suppressed and, thus, a better ground layer surface can beprovided as compared with a nitride semiconductor containing no Al suchas GaN, whereby, it is preferable to use a nitride semiconductorcontaining Al.

(Crack Preventing Layer 104)

Then, TMG, TMI (trimethylindium) and ammonia are used to grow a crackpreventing layer 104 comprising In_(0.06)Ga_(0.94)N at a thickness of0.15 μm at a temperature of 800° C. This crack preventing layer may beomitted.

(N-Side Cladding Layer 105) (Lower Cladding Layer 25))

Then, TMA, TMG and ammonia as a raw material are used to grow an A layercomprising undoped Al_(0.14)Ga_(0.86)N at a thickness of 25 Å at atemperature of 1050° C. Subsequently, TMA is stopped, a silane gas isused as impurity gas to grow a B layer comprising GaN doped with Si at5×10¹⁸/cm³ at a thickness of 25 Å. A procedure of laminating an A layerand a B layer alternately is repeated 120 times to laminate an A layerand a B layer, to grow an n-side cladding layer 106 comprising amulti-layered membrane (superlattice structure) having a total thicknessof 0.6 μm.

(N-Side Light Guiding Layer 106 (First Light Guiding Layer 26))

Then, at the same temperature, TMG and ammonia are used as a rawmaterial gas to alternately laminate an A layer comprising Si-doped GaNof thickness of 25 Å and a B layer comprising Al_(0.06)Ga_(0.95)N of athickness of 25 Å (by adding TMA as a raw material gas) 30 times, togrow an n-side light guiding layer 106 comprising a superlatticemulti-layered membrane having a thickness of a 0.15 μm.

(Active Layer 107)

Then, as shown in FIG. 7, TMI (trimethylindium), TMG and TMA are used asa raw material gas to laminate a barrier layer comprising Si-dopedAl_(0.1)Ga_(0.9)N and a well layer comprising undopedIn_(0.03)Al_(0.02)Ga_(0.95)N thereon in an order of barrier layer 2a/well layer 1 a/barrier layer 2 b/well layer 1 b/barrier layer 2 c at atemperature of 800° C. Upon this, as shown in FIG. 7, the barrier layer2 a is formed at a thickness of 200 Å, the barrier layers 2 b and 2 care formed at a thickness of 40 Å, and the well layers 1 a and 1 b areformed at a thickness of 70 Å. An active layer 107 has a multiplequantum well structure (MQW) having a total thickness of about 420 Å.

(P-Side Electron Confining Layer 108 (Carrier Confining Layer 28))

Then, at the same temperature, TMA, TMG and ammonia are used as a rawmaterial gas, and Cp₂Mg (cyclopentadienylmagnesium) is used as animpurity gas to grow a p-side electron confining layer 108 comprisingAl_(0.3)Ga_(0.7)N doped with Mg at 1×10¹⁹/cm³ at a thickness of 10 mm.This layer needs not to be provided. However, by provision, this layerfunctions to confine electrons, and contributes to decrease in athreshold.

(P-Side Light Guiding Layer 109 (Second Light Guiding Layer 29))

Then, at a temperature of 1050° C., TMG and ammonia are used as a rawmaterial gas to alternately laminate an A layer comprising Mg-doped GaNof a thickness of 25 Å and a B layer comprising Al_(0.06)Ga_(0.94)N of athickness of 25 Å (by adding TMA as a raw material) 20 times, to grow ap-side light guiding layer 109 having a superlattice multi-layeredstructure of a thickness of 0.15 μm.

Since this p-side light guiding layer 109 is doped with Mg by diffusionof Mg from an adjacent layer such as the p-side electron confining layer108, the p-side cladding layer 109 and the like, even when formedundoped, the guiding layer 109 can be rendered a Mg-doped layer.

(P-Side Cladding Layer 110 (Upper Cladding Layer 30))

Subsequently, at 1050° C., an A layer comprising undopedAl_(0.14)Ga_(0.86)N is grown at a thickness of 25 Å. Subsequently, Cp₂Mgis used to grow a B layer comprising Mg-doped Al_(0.14)Ga_(0.86)N at athickness of 25 Å. A procedure of laminating an A layer and a B layeralternately is repeated 100 times to grow a p-side cladding layer 110comprising a superlattice multi-layered membrane of a total thickness of0.5 μm.

(P-Side Contact Layer 111)

Finally, at 1050° C., a p-side contact layer 111 comprising a p-type GaNdoped with Mg at 1×10²⁰/cm³ is grown at a thickness of 150 Å on a p-sidecladding layer 110. The p-side contact layer 111 may be composed ofp-type In_(X)Al_(Y)Ga_(1−X−Y)N (0≦X, 0≦Y, X+Y≦1), preferably, GaN dopedwith p-type impurity, or AlGaN having an Al ratio of 0.3 or smaller.Whereby, most preferable ohmic contact with a p-electrode 120 isobtained and, most preferably, by adopting GaN, best ohmic contactbecomes possible. Since the contact layer 111 is a layer on which anelectrode is formed, the high carrier concentration of 1×10¹⁷/cm³ orgreater is desirable. When the concentration is less than 1×10¹⁷/cm³,there is a tendency that it becomes difficult to obtain preferable ohmiccontact with an electrode. Further, by adopting GaN as a composition fora contact layer, preferable ohmic contact with an electrode material iseasily obtained. After completion of a reaction, a wafer is annealed ina reactor in the nitrogen atmosphere at 700° C., to render a p-typelayer lower resistant.

After a nitride semiconductor is grown to laminate respective layers asdescribed above, a wafer is removed from a reactor, a protectingmembrane comprising SiO₂ is formed on the surface of an uppermost p-sidecontact layer, which is etched with SiCl₄ gas using RIE (reactive ionetching) to expose the surface of an n-side contact layer 103 on whichan n-electrode is to be formed as shown in FIG. 1. Like this, in orderto etch a nitride semiconductor deep, SiO₂ is best as a protectingmembrane.

Then, as the aforementioned stripe-like waveguide region, ridge stripeis formed. First, a first protecting membrane 161 comprising a Si oxide(mainly SiO₂) is formed at a thickness of 0.5 μm on the almost surfaceof an uppermost p-side contact layer (upper contact layer) using a PVDapparatus, a mask having a prescribed shape is placed on a firstprotecting membrane, which is subjected to the photolithographytechnique with a RIE (reactive ion etching) apparatus using CF₄ gas toobtain a first protecting membrane 161 having a stripe width of 1.6 μm.Upon this, a height of ridge stripe (etching depth) is formed by etchinga part of the p-side contact layer 111, the p-side cladding layer 109,and the p-side light guiding layer 110, and etching the p-side lightguiding layer 109 to a thickness of 0.1 μm.

Then, after formation of ridge stripe, a second protecting membrane 162comprising a Zr oxide (mainly ZrO2) is continuously formed at athickness of 0.5 μm on the first protecting membrane 161 and a p-sidelight guiding layer 109 exposed by etching.

After formation of the second protecting membrane 162, a wafer isheat-treated at 600° C. When a material other than SiO₂ is formed as asecond protecting membrane like this, after formation of the secondprotecting membrane, by heat treatment at a temperature of not lowerthan 300° C., preferably not lower than 400° C. and not higher than adegrading temperature of a nitride semiconductor (1200° C.), the secondprotecting membrane becomes difficult to be solved in a material(hydrofluoric acid) for dissolving the first protecting membrane and,therefore, it is more desirable to add this step.

Then, a wafer is soaked in hydrofluoric acid to remove the firstprotecting membrane 161 by a lift off method. This removes the firstprotecting membrane 161 provided on the p-side contact layer 111, toexpose a p-side contact layer. Like this, as shown in FIG. 1, the secondprotecting membrane (embedding layer) 162 is formed on a side of ridgestripe and sequential plane (exposed plane of the p-side light guidinglayer 109).

Like this, after the first protecting membrane 161 provided on thep-side contact layer 112 is removed, as shown in FIG. 1, the p-electrode120 comprising Ni/Au is formed on the surface of the exposed p-sidecontact layer 111. The p-electrode 120 has a stripe width of 100 μm and,as shown in FIG. 1, is formed over the second protecting membrane 162.After formation of the second protecting membrane 162, the stripe-liken-electrode 121 comprising Ti/Al is formed on the surface of the alreadyexposed n-side contact layer 103 parallel with stripe.

Then, in order to provide a take out electrode in a p, n-electrode on aside exposed by etching to form an n-electrode, a desired region ismasked, a dielectric multi-layered membrane 164 comprising SiO₂ and TiO₂is provided thereon, and take out (pat) electrodes 122 and 123comprising Ni—Ti—Au (1000 Å-1000 Å-8000 Å) are provided on a p,n-electrode, respectively. Upon this, the width of the active layer 107is 200 μm (width in a direction vertical to a resonator direction), anda dielectric multi-layered membrane comprising SiO₂ and TiO₂ is providedon the surface of a resonator (reflective side). Like this, after then-electrode and the p-electrode are formed, the M plane of a nitridesemiconductor (M plane of GaN, such as (11-00)) is divided into bar-likein a direction vertical to a stripe-like electrode, and a bar-like waferis further divided to obtain a laser device. Upon this, a length of aresonator is 650 μm.

Upon conversion into bar-like, cleavage is performed in a waveguideregion held by etching ends, and the resulting cleaved plane may be aresonator plane. Alternatively, cleavage is performed outside awaveguide region, and an etched end may be a resonator plane. A pair ofresonator planes having one plane as an etched end and other plane as acleaved plane may be formed. In addition, a reflective membranecomprising a dielectric multi-layered membrane is provided on aresonating plane of the aforementioned etched end. Alternatively, areflective membrane may be provided also on a resonator plane of acleaved plane after cleavage. Upon this, as a reflective membrane, atleast one selected from the group consisting of SiO₂, TiO₂, ZrO₂, ZnO,Al₂O₃, MgO and polyimide is used. Alternatively, a multi-layeredmembrane obtained by laminating at a thickness of λ/4n (λ is awavelength, and n is a refractive index of a material) may be used, oronly one layer may be used, or a reflective membrane functions also as asurface protecting membrane for preventing a resonator end fromexposing. In order to function as a surface protecting membrane, amembrane is formed at a thickness of λ/2n. In addition, an etched end isnot formed in a device processing step, that is, only an n-electrodeforming plane (n-side contact layer) is exposed, and a laser devicehaving a pair of cleaved planes as a resonator plane may be obtained.

Also upon further division of a bar-like wafer, a cleaved plane of anitride semiconductor (single substrate) may be used. A nitridesemiconductor (GaN) which is vertical to a cleaved plane when cleavedinto bar-like may be cleaved at an M plane or an A plane ({1010}) whichis approximated by hexagonal system, and a chip may be taken out.Alternatively, upon cleaved into bar-like, an A plane of a nitridesemiconductor may be used.

The resulting laser device is a nitride semiconductor device which iscontinuously oscillated at a wavelength of 370 nm at room temperature.In addition, an n-side or p-side light guiding layer is composed ofAlGaN having an Al average ratio of 0.03, and a waveguide is formed inwhich a difference between a band gap energy E_(g) of a first lightguiding layer and a second light guiding layer and the photon energyE_(p) of laser light (emitting wavelength of an active layer),E_(g)−E_(p), is 0.05 eV or greater.

EXAMPLE 2

According to the same manner as that of Example 1 except that an activelayer is formed as described below in Example 1, a laser device isobtained.

(Active Layer 107)

A barrier layer comprising Si-doped In_(0.01)Al_(0.1)Ga_(0.89)N, and awell layer comprising undoped In_(0.03)Al_(0.02)Ga_(0.95)L thereon arelaminated in an order of barrier layer 2 a/well layer 1 a/barrier layer2 b/well layer 1 b/barrier layer 2 c. Upon this, as shown in FIG. 7, thebarrier layer 2 a is formed at a thickness of 200 Å, the barrier layers2 b and 2 c are formed at a thickness of 40 Å, and the well layers 1 aand 1 b are formed at a thickness of 70 Å. An active layer 107 has amultiple quantum well structure (MQW) having a total thickness of about420 Å.

The resulting laser device is a nitride semiconductor device which iscontinuously oscillated at a wavelength of 370 nm at room temperature asin Example 1.

EXAMPLE 3

According to the same manner as that of Example 1 except that an activelayer, a light guiding layer and a cladding layer are formed asdescribed below in Example 1, a laser device is obtained.

(N-Side Cladding Layer 105 (Lower Cladding Layer 25))

A procedure of alternately laminating an Å layer comprising undopedAl_(0.3)Ga_(0.7)N at a thickness of 25 Å and a B layer comprisingAl_(0.2)Ga_(0.8)N doped with Si at 5×10¹⁸/cm³ at a thickness of 25 Å isrepeated 120 times to laminate an A layer and a B layer, to form ann-side cladding layer 106 comprising a multi-layered membrane(superlattice structure) at a total thickness of 0.6 μm.

(N-Side Light Guiding Layer 106 (First Light Guiding Layer 26))

An A layer comprising Si-doped Al_(0.1)Ga_(0.9)N having a thickness of25 Å and a B layer comprising Al_(0.03)Ga_(0.1)N having a thickness of25 Å are alternately laminated 30 times to grow an n-side light guidinglayer 106 comprising a superlattice multi-layered membrane having athickness of 0.15 μm.

(Active Layer 107)

A barrier layer comprising Si-doped Al_(0.2)Ga_(0.8)N and a well layercomprising undoped In_(0.03)Al_(0.02)Ga_(0.95)N thereon are laminated inan order of barrier layer 2 a/well layer 1 a/barrier layer 2 b/welllayer 1 b/barrier layer 2 c. Upon this, as shown in FIG. 7, the barrierlayer 2 a is formed at a thickness of 200 Å, the barrier layers 2 b and2 c are formed at a thickness of 40 Å, and the well layers 1 a and 1 bare formed at a thickness of 70 Å. An active layer 107 becomes amultiple quantum well structure (MQW) having a total thickness of about420 Å.

(P-Side Light Guiding Layer 109 (Second Light Guiding Layer 29))

An A layer comprising Mg-doped Al_(0.1)Ga_(0.9)N having a thickness of25 Å and a B layer comprising Al_(0.1)Ga_(0.9)N having a thickness of 25Å are alternately laminated 30 times to grow a p-side light guidinglayer 109 of a superlattice multi-layered structure having a thicknessof 0.15 μm.

(P-Side Cladding Layer 110 (Upper Cladding Layer 30))

An A layer comprising undoped Al_(0.3)Ga_(0.7)N is grown at a thicknessof 25 Å, and a B layer comprising Mg-doped Al_(0.1)Ga_(0.9)N is grown ata thickness of 25 Å, and a procedure of alternately laminating An Alayer and a B layer is repeated 100 times to grow a p-side claddinglayer 110 comprising a superlattice multi-layered membrane having atotal thickness of 0.5 μm.

The resulting laser device is a nitride semiconductor device which iscontinuously oscillated at a shorter wavelength region than Example 1, awavelength of 350 nm, at room temperature. An n-side or p-side lightguiding layer is composed of AlGaN having an Al average ratio of 0.2. Awaveguide is formed in which a difference between a band gap energyE_(g) of a first light guiding layer and a second light guiding layerand a photon energy E_(p) of laser light, E_(g)−E_(p), is 0.05 eV orgreater.

EXAMPLE 4

According to the same manner as that of Example 1 except that respectivelight guiding layers are formed as described below in Example 1, a laserdevice is obtained.

(N-Side Light Guiding Layer 106 (First Light Guiding Layer 26))

An n-side light guiding layer 106 comprising Si-dopedAl_(0.03)Ga_(0.97)N (Al average ratio of Example 1) is formed at athickness of 0.15 μm, to provide a single membrane light guiding layer.

(P-Side Light Guiding Layer 109 (Second Light Guiding Layer 29))

A p-side light guiding layer 109 comprising Mg-doped Al_(0.03)Ga_(0.97)N(Al average ratio of Example 1) is formed at a thickness of 0.15 μm, toprovide a single membrane light guiding layer.

Although the resulting laser device has the same Al average ratio ascompared with Example 1, there is a tendency that the crystallizingproperty is deteriorated due to provision of a single membrane lightguiding layer. In addition, since a doping region of a light guidinglayer is large, there is a tendency that the light loss occurs greatlydue to impurity doping and a threshold current density is increased.

In addition, as another single membrane light guiding layer, in Example1, a p-side light guiding layer and an n-side light guiding layer areformed of undoped Al_(0.035)Ga_(0.965)N at a thickness of 75 nm to forman active layer having a single quantum well structure in which abarrier layer 2 c and a well layer 1 b are not provided. In theresulting laser device, since a light guiding layer is a single membraneas compared with Example 1, the crystallizing property is deteriorated.On the other hand, by adopting about half a thickness, deterioration ofa device due to deterioration of the crystallizing property can besuppressed. In addition, by forming a light guiding layer undoped, astructure is obtained in which the light loss in a waveguide issuppressed. In addition, since an active layer has a single quantum wellstructure, by suppressing deterioration of the crystallizing property inan active layer, a laser device approximately equivalent to that ofExample 1 is obtained. Here, undoped indicates that doping is notperformed intentionally at growth. As described above, a p-side lightguiding layer is slightly doped with Mg due to Mg diffusion from anadjacent p-side electron confining layer and p-side cladding layer. Siis slightly diffused, whereby, an n-side light guiding layer becomesundoped. In a nitride semiconductor, since a diffusion length for ann-type carrier is longer as compared with p-type, and an undoped nitridesemiconductor shows n-type although high resistant due to N atomvacancy, even when converted into undoped n-side light guiding layer,carriers can be injected into an active layer.

EXAMPLE 5

According to the same manner as that of Example 1 except that a lightguiding layer is formed by gradient composition as described below asshown in FIG. 4 in Example 1, a laser device is obtained.

(N-Side Light Guiding Layer 106 (First Light Guiding Layer 26))

Al_(x)Ga_(1−x)N is formed at a thickness of 0.15 μm. Upon this, an Alratio x is changed from 0.05 to 0.01 with growing, to provide an n-sidelight guiding layer 106 having gradient composition in a thickness ofdirection. Upon this, an n-side light guiding layer is formed by Sidoping at a region of a first thickness of 50 nm and is formed undopedat a remaining region of a thickness of 0.1 μm (region of 0.1 μm on anactive layer side).

(P-Side Light Guiding Layer 109 (Second Light Guiding Layer 29))

Al_(x)Ga_(1−x)N is formed at a thickness of 0.15 μm. Upon this, an Alratio x is changed from 0.01 to 0.05 with growing, to provide a p-sidelight guiding layer 109 having gradient composition in a thicknessdirection. Here, a p-side light guiding layer is formed undoped at afirst thickness of 0.1 μm (region of 0.1 μm on an active layer side) andis formed by Mg doping at a remaining region of a thickness of 50 nm.

The resulting laser device has almost the same Al average ratio ascompared with Example 1. However, as shown in FIG. 4, by providing alight guiding layer having gradient band gap energy, effects ofinjecting carriers in an active layer becomes better and the internalquantum efficacy is improved. In addition, since an undoped region isprovided on a side near an active layer in a light guiding layer (activelayer side), a waveguide structure is obtained in which the light lossdue to impurity doping is suppressed low, and there is a tendency that athreshold current density is decreased.

EXAMPLE 6

According to the same manner as that of Example 1 except that a lightguiding layer is formed by gradient composition as described below asshown in FIG. 4 in Example 5, a laser device is obtained.

(N-Side Light Guiding Layer 106 (First Light Guiding Layer 26))

An A layer comprising Al_(x)Ga_(1−x)N having a thickness of 25 Å and a Blayer comprising Al_(y)Ga_(1−y)N (x>y) having a thickness of 25 Å arealternately laminated 30 times to form an n-side light guiding layer ofa superlattice multi-layered structure having a thickness of 0.15 μm.Upon this, an Al ratio x is changed from 0.05 to 0.03 with growing, andan Al ratio y is constant at 0.015, to provide an n-side light guidinglayer 106 having gradient composition. Upon this, in an n-side lightguiding layer, both the A layer and the B layer are formed by Si dopingat a first thickness of 50 nm, and only the A layer is formed by Sidoping and the B layer is formed undoped in a modified doping manner ata remaining region of a thickness of 0.1 μm (region of 0.1 μm on anactive layer side).

(P-Side Light Guiding Layer 109 (Second Light Guiding Layer 29))

An A layer comprising Al_(y)Ga_(1−x)N having a thickness of 25 Å and a Blayer comprising Al_(y)Ga_(1−y)N (x>y) having a thickness of 25 Å arealternately laminated 30 times to form a p-side light guiding layer 109of a superlattice multi-layered structure having a thickness of 0.15 μm.Here, in a p-side light guiding layer, the A layer is formed by Mgdoping and the B layer is formed undoped at a first thickness of 0.1 μm(region of 0.1 μm on an active layer side), and both the A layer and theB layer are formed by Mg doping at a remaining region of a thickness of50 nm.

The resulting laser device has almost the same Al average ratio ascompared with Example 4. However, the crystallizing property becomesbetter due to superlattice structure and the device properties areimproved. On the other hand, since an undoped region for a light guidinglayer is small as compared with Example 4, there is a tendency that thelight loss becomes larger and a threshold current density is slightlyincreased.

EXAMPLE 7

An end emitting laser device shown in FIG. 8 will be explained below.

(Substrate 501)

The same substrate 501 as the nitride semiconductor substrate 101 usedin Example 1 is used.

Each three layers of a first layer 531 comprising Al_(x)Ga_(1−x)N(0≦x≦1) and a second layer 532 comprising Al_(y)Ga_(1−y)N (0<y≦1, x<y)having a different composition from that of the first layer arealternately laminated as a reflective membrane 530 on the nitridesemiconductor substrate 501. Upon this, respective layers are providedat such a thickness that satisfies an equation: λ/(4n)(λ is a lightwavelength, and n is a refractive index of a material). As a reflectivemembrane of a nitride semiconductor, a multi-layered membrane in whichfirst and second layers and a nitride semiconductor represented byAl_(x)Ga_(1−x)N (0≦x≦1) having a different composition are alternatelylaminated, may be used. Upon this, one or more of respective layers, andone or more pairs of first layer/second layer are formed. Specifically,first layer/second layer may be formed of AlGaN/AlGaN, GaN/AlGaN,AlGaN/AlN, GaN/AlN or the like. In the case ofAl_(x)Ga_(1−x)N/Al_(y)Ga_(1−y)N (0<x, x<y<1), since it is an AlGaNmulti-layered membrane, a difference in a thermal expansion coefficientcan be rendered small and it can be formed with the better crystallizingproperty. In the case of GaN/Al_(y)Ga_(1−y)N (0<y<1), a multi-layeredmembrane having the improved crystallizing property can be obtained dueto a GaN layer. In addition, when a difference in an Al ratio (y−x) isrendered large, a difference in a refractive index between the firstlayer and the second layer becomes large and a reflective rate becomeshigh. Specifically, adopting y−x≧0.3, preferably y−x≧0.5, amulti-layered reflective membrane having a high reflective rate can beformed. In addition, as in Example 1, by forming Al_(y)Ga_(1−y)N (0<y≦1)as a multi-layered membrane, it functions as a buffering layer 102, andthe pit reducing effects can be obtained. In addition, a reflectivemembrane can be provided between a substrate and an active layer, or onan active layer. The aforementioned dielectric multi-layered membranecan be applied to a reflective membrane on an active layer. Further, itcan be applied to an end emitting light emitting device in which areflective membrane is divided between a substrate and an active layer.

Subsequently, under the same conditions as those of Example 1, an n-sidecontact layer 533, an active layer 534, a p-side electron confininglayer (not shown) and a p-side contact layer 535 are laminated toprovide a block layer 536 comprising SiO₂ having a circular opening.Mg-doped GaN is grown from the circular opening to form a second p-sidecontact layer 537. Upon this, either of a p-side contact layer 535 or asecond p-side contact layer 537 may be formed. A dielectricmulti-layered membrane comprising SiO₂/TiO₂ is formed on the secondp-side contact layer 537 to obtain a reflective membrane 538, which isprovided on an opening of the aforementioned block layer 536 in thecircular shape. And, etching is performed to such a depth that then-side contact layer 533 is exposed, and a ring-like n-electrode 521 isformed on the exposed n-side contact layer 533, and a p-electrode 520surrounding the reflective membrane 538 is formed on the second p-sidecontact layer 537, respectively. An end emitting laser device thusobtained is a laser device which is oscillated at a short wavelength asin Example 1.

EXAMPLE 8

According to the same manner as that of Example 1 except that a claddinglayer, a light guiding layer and an active layer are formed as describedbelow in Example 1, a laser device having a device structure isobtained.

As upper and lower cladding layers, Al_(0.1)Ga_(0.9)N having a thicknessof 25 Å and Al_(0.05)Ga_(0.95)N having a thickness of 25 Å arealternately laminated 100 times to obtain a superlattice multi-layeredstructure (500 Å). Upon this, in p-side and n-side cladding layers, oneof a superlattice layer is doped with Mg and Si as a dopant,respectively.

As upper and lower light guiding layers, undoped Al_(0.04)Ga_(0.96)N isformed at a thickness of 0.15 μm.

A barrier layer comprising Al_(0.15)In_(0.01)Ga_(0.84)N (200 Å), a welllayer having a thickness of 100 Å and a barrier layer comprisingAl0.15In_(0.01)Ga_(0.84)N (45 Å) are laminated as an active layer toobtain a quantum well structure.

Dependency of an Al crystal mixing ratio x (x=0.03, 0.06, 0.08) shown inFIG. 10 shows a threshold current density and a change in a wavelengthunder pulse oscillation using Al_(x)In_(0.04)Ga_(0.96-x)N as a welllayer. Dependency of an In crystal mixing ratio y (y=0.02, 0.03, 0.04,0.07) shown in FIG. 11 shows a threshold current density and a change ina wavelength under pulse oscillation using Al_(0.03)In_(y)Ga_(0.97-y)N.

As shown in FIG. 11, in a threshold current density J_(th), as an Incrystal mixing ratio y grows larger from around 0.02, a falling curve isshown, a minimum value is seen in a range of 0.03 to 0.05 and, in aregion exceeding 0.05, an increasing tendency is shown. In addition,regarding an Al crystal mixing ratio x, as shown in FIG. 10, in a rangeof x≦0.1, there is an increasing tendency with increase in an Al crystalmixing ratio x and, in a range of 0<x≦0.6, a threshold current can bepreferably decreased.

EXAMPLE 9

The details of a structure of a ridge-type nitride semiconductor lasershown in FIG. 1 are explained below.

A n-type contact layer 103, a crack preventing layer 104, a n-typecladding layer 105 and a n-type light guiding layer 106 which are an-type nitride semiconductor layer, are formed on a substrate 101 via abuffering layer 102. Other layers except for a n-type cladding layer 105can be omitted depending upon an device. It is necessary that a n-typenitride semiconductor layer has the wider band gap than that of anactive layer at least its part in contact with an active layer. For thisreason, a composition containing Al is preferable. In addition, eachlayer may be a n-type by grown while doping with n-type impurity, or maybe n-type grown undoped.

An active layer 107 is formed on n-type nitride semiconductor layers103˜106. As described above, an active layer 107 has a MQW structure inwhich an Al_(x1)Ga_(1−x2)N well layer (0≦x1<1) and an Al_(x2)Ga_(1−x2)Nbarrier layer (0<x2<1, x1<x2) are repeatedly laminated alternatelysuitable times, and there are a barrier layer at both ends of an activelayer. A well layer is formed undoped, and all barrier layers are dopedwith n-type impurity such as Si, Sn or the like preferably at theconcentration of 1×10¹⁷˜1×10¹⁹ cm⁻³.

A final barrier layer is formed undoped, and contains p-type impuritysuch as Mg or the like diffused from a p-type electron confining layer108 which is to be grown next, at the concentration of 1×10¹⁶˜1×10¹⁹cm⁻³ in some cases. A final barrier layer may be grown while doping withp-type impurity such as Mg or the like at the concentration of 1×10¹⁹cm⁻³ or smaller. Like other barrier layers, a final barrier layer may bedoped with n-type impurity such as Si.

A p-type electron confining layer 108, a p-type light guiding layer 109,a p-type cladding layer 110 and a p-type contact layer 111 as a p-typenitride semiconductor layer are formed on a final barrier layer. Otherlayers except for a p-type cladding layer may be omitted depending uponan device. It is necessary that a p-type nitride semiconductor layer hasthe wider band gap than that of an active layer at least at its part incontact with an active layer. For that reason, a composition containingAl is preferable. Each layer may be rendered a p-type by growing whiledoping with p-type impurity, or may be rendered p-type by diffusingp-type impurity from an adjacent other layer.

A p-type electron confining layer 108 comprises a p-type nitridesemiconductor having a higher Al crystal mixing ratio than that of ap-type cladding layer 110, and preferably has a composition ofAl_(x)Ga_(1−x)N (0.1<x<0.5). In addition, the layer 108 may be dopedwith p-type impurity such as Mg or the like at the high concentration,preferably at the concentration of 5×10¹⁷˜1×10¹⁹ cm⁻³. Whereby, a p-typeelectron confining layer 108 can effectively confine electrons into anactive layer, leading to decrease in a threshold for laser. In addition,a p-type electron confining layer 108 may be grown at a thickness ofaround 30˜200 Å and, when a thickness is smaller a layer 108 may begrown at a lower temperature than that for a p-type light guiding layer109 or a p-type light cladding layer 110.

In addition, a p-type electron confining layer 108 plays a role insupplying p-type impurity to a final barrier layer grown in undoped bydiffusion. Both cooperate to protect an active layer 107 from degradingand, at the same time, may have a role in enhancing the efficacy forinjecting holes into an active layer 107.

Among p-type nitride semiconductor layers, ridge stripe is formed to amid way of a p-type light guiding layer 109, and protecting membranes161 and 162, a p-type electrode 120, a n-type electrode 121, a p-patelectrode 121 and n-pat electrode 123 are formed to construct asemiconductor laser.

The present Example has same structure and manufacturing process as inExample 1, the composition of the quantum well layer in the active layer107 is changed into ternary material of AlGaN as below in order to set aoscillation wavelength (366 nm) shorter than the oscillation wavelength(370 nm) of Example 1. Further, each composition of the n-side claddinglayer 105, the n-side light guiding layer 106, the p-side light guidinglayer 109 and the p-side cladding layer 110 is also changed as follows.

(N-Side Cladding Layer 105: Lower Cladding Layer 25)

Then, a temperature is risen to 1050° C., TMA, TMG and ammonia as a rawmaterial gas, and silane gas as impurity gas are used to grow an A layercomprising Al_(0.05)Ga_(0.95)N doped with Si at 5×10¹⁸/cm³ at athickness of 25 Å. Subsequently, the impurity gas is stopped. A B layercomprising undoped Al_(0.1)Ga_(0.9)N is grown at a thickness of 25 Å.Then, this procedure is repeated 100 times to laminate an A layer and aB layer to grow a n-type cladding layer 106 comprising a multi-layeredmembrane (superlattice structure). Upon this, when an Al crystal mixingratio for undoped AlGaN is in a range of not less than 0 05 and notgreater than 0.3, a difference in a refractive index which functionssufficiently as a cladding layer can be set.

(N-Side Light Guiding Layer 106: Lower Light Guiding Layer 26)

Then, at the same temperature, TMA, TMG and ammonia as a raw materialgas are used to grow a n-type light guiding layer 106 comprising undopedAl_(0.05)Ga_(0.95)N at a thickness of 0.15 μm. Alternatively, n-typeimpurity may be doped therein. This layer serves as a first nitridesemiconductor layer.

(Active Layer 107 (27, 12))

Then, at the same temperature, TMA, TMG and ammonia as a raw materialgas, and silane gas as impurity gas are used to laminate a barrier layer(first barrier layer 2 a) comprising Al_(0.15)Ga_(0.8) ₅N doped with Siat 5×10¹⁸/cm³ (B) at a thickness of 100 Å and, after stoppage of TMA andsilane gas, a well layer comprising undoped GaN (W) at a thickness of100 Å and, as a final barrier layer (second barrier layer 2 b), undopedAl_(0.15)Ga_(0.85)N at a thickness of 45 Å in an order of (B)/(W)/(B).An active layer 107 may be rendered a multiple quantum well structure(MQW) by repeating lamination in an order of (B)/(W)/(B).

(P-Side Light Guiding Layer 109: Upper Light Guiding Layer 29)

Then, a temperature is maintained at 1050° C., TMA, TMG and ammonia as araw material gas are used to grow a p-side light guiding layer 109comprising undoped Al_(0.05)Ga_(0.95)N at a thickness of 0.15 μm. Thisp-type light guiding layer 109 is grown undoped, and the layer 109 hasthe Mg concentration of 5×10¹⁶/cm³ by diffusion of Mg from an adjacentlayer such as a p-type electron confining layer 108, a p-type claddinglayer 110 and the like, indicating p-type. Alternatively, this layer maybe intentionally doped with Mg at growth. This layer serves as a secondnitride semiconductor layer

(P-Side Cladding Layer 110: Upper Cladding Layer 30)

Subsequently, TMA is stopped, Cp₂Mg is to grow a layer comprisingMg-doped Al_(0.05)Ga_(0.95)N at a thickness of25 Å at 1050° C.Subsequently, Cp₂Mg is stopped, and a layer comprising undopedAl_(0.10)Ga_(0.90)N is grown at a thickness of 25 Å. This is repeated100 times to grow a p-type cladding layer 115 comprising a superlatticelayer having a total thickness of 0.45 μm. When made of a superlatticein which at least one of p-type cladding layers comprises a nitridesemiconductor layer containing Al, and nitride semiconductor layershaving a different band gap energy, any one of layers is doped withimpurity at a greater amount to effect so-called modified doping,whereby, there is a tendency that the crystallizing property becomesbetter. Alternatively, both layers may be doped in the same manner.Since an Al crystal mixing ratio for a hole cladding layer can beincreased by allowing a p-side cladding layer 110 to have a superlatticestructure, a refractive index of a cladding layer itself is decreasedand, further, the band gap energy is increased, being very effective indecreasing a threshold, further, superlattice decreases pit produced ina cladding layer itself as compared with no superlattice, reducingoccurrence of short circuit.

A continuously oscillating laser device having a threshold current of 53mA, a current density of 3.5 kA/cm² and an oscillating wavelength of 366nm at room temperature is obtained.

EXAMPLE 10

The present Example is as same as Example 9 except that an active layeris described below, a laser device is obtained.

(Active Layer 107 (27, 12))

An active layer having a single quantum well structure used is obtainedby laminating a first barrier layer 2 a comprising Si-dopedAl_(0.15)Ga_(0.85)N having a thickness of 200 Å, a well layer 1 bcomprising undoped GaN having a thickness of 100 Å and a second barrierlayer 2 b comprising undoped Al_(0.l5)Ga_(0.85)N having a thickness of45 Å successively.

The resulting laser device has a tendency that the crystallizingproperty of a well layer is deteriorated due to increase in a thicknessof a first barrier layer, and there is a tendency that threshold currentJ_(th) is increased to 100 mA.

EXAMPLE 11

The present Example is as same as Example 9 except that an active layeris described below, a laser device is obtained.

(Active Layer 27)

An active layer having a single quantum well structure used is obtainedby laminating a first barrier layer 2 a comprising Si-dopedAl_(0.15)Ga_(0.85)N having a thickness of 100 Å, a well layer 1 b havinga thickness of 100 Å and a second barrier layer 2 b comprising undopedAl_(0.15)Ga_(0.85)N having a thickness of 150 Å successively.

The resulting laser device has a tendency that Vf is increased due toincrease in a thickness of a second barrier layer as compared withExample 1, and there is a tendency that a threshold current J_(th) isincreased to 100 Ma. Here, a structure is obtained in which a secondbarrier has a greater thickness (100 Å or greater) than that of a firstbarrier layer, a p-side electron confining layer in the vicinity of ap-n junction has high resistance, and a layer producing the great heatis isolated from a well layer with a second barrier layer, whereby,influence of a second barrier layer can be increased and, on the otherhand, increase in resistance due to a second barrier layer isinfluenced.

EXAMPLE 12

The present Example is as same as Example 9 except that an active layeris described below, a laser device is obtained.

(Active Layer 27)

An active layer having a single quantum well structure used is obtainedby laminating a first barrier layer 2 a comprising Si-dopedAl_(0.05)Ga_(0.95)N having a thickness of 100 Å, a well layer 1 bcomprising undoped GaN having a thickness of 100 Å and a second barrierlayer 2 b comprising undoped Al_(0.05)Ga_(0.95)N having a thickness of150 Å successively.

The resulting laser device has a tendency that, since an Al crystalmixing ratio in a first barrier layer and a second layer is renderedsmaller, the band gap energy is rendered smaller, and a difference inthe band gap energy from a well layer is rendered smaller as comparedwith Example 1, confinement of carriers into a well layer isdeteriorated, and a threshold current J_(th) is increased to 200 mA.Here, a difference in an Al crystal mixing ratio between a well layerand a barrier layer (first barrier layer), X_(B1)−X_(W) is 0.05. Byusing this Al crystal mixing ratio difference as a boundary andincreasing a difference in an Al crystal mixing ratio between a barrierlayer and a well layer, there is a tendency that a threshold isdecreased.

EXAMPLE 13

The present Example is as same as Example 9 except that a light guidinglayer is formed by gradient ratio as described below as shown in FIG.6B, a laser device is obtained.

(N-Side Light Guiding Layer 106 (First Layer Guiding Layer 26))

By forming Al_(x)Ga_(1−x)N at a thickness of 0.15 μm and, upon this,varying an Al ratio x from 0.1 to 0.02 as growing, that is, asapproaching an active layer, a n-side light guiding layer 106 having agradient ratio in a thickness direction is provided. Upon this, a n-sidelight guiding layer is formed by Si doping at a first region of athickness of 50 nm and formed undoped at a remaining region of athickness of 0.1 μm (region of a thickness of 0.1 μm on an active layerside). Here, in a light guiding layer in the vicinity of an activelayer, a part region having the smaller band gap energy than that of afirst barrier layer becomes a first nitride semiconductor layer.

(P-Side Light Guiding Layer 109 (Second Light Guiding Layer 29))

By forming Al_(x)Ga_(1−x)N at a thickness of 0.15 μm and, upon this,varying an Al ratio x from 0.02 to 0.1 with growing to form a ratiogradient in a thickness direction, a p-side light gradient layer 109 isprovided in which an Al crystal mixing ratio becomes smaller and theband gap energy becomes smaller as approaching an active layer. Here, ap-side light guiding layer is formed undoped at a first region of athickness of 0.1 μm (region of a thickness of 0.1 μ m on an active layerside) and is formed by doping with Mg at a remaining region of athickness of 50 nm. Upon this, in a p-side light guiding layer 29 in thevicinity of an active layer and a p-side electron confining layer, apart region having a smaller Al crystal mixing ratio and band gap energythan those of a second barrier layer becomes a second nitridesemiconductor layer.

The resulting laser device has approximately the same Al average ratioas compared with Example 1. However, as shown in FIG. 4, by provision ofa light guiding layer having gradient band gap energy, there is atendency that the efficacy of injecting carriers into an active layerbecomes better and the internal quantum efficacy is improved. Inaddition, since an undoped region is provided on a side near an activelayer (active layer side) in a light guiding layer, a waveguidestructure is obtained by which the light loss due to impurity doping issuppressed low, and, there is a tendency that a threshold currentdensity is decreased.

EXAMPLE 14

The present Example is as same as Example 9 except that a light guidinglayer is formed by gradient ratio as described below as shown in FIG.6B, a laser device is obtained.

(N-side Light Guiding Layer 106 (First Light Guiding Layer 26))

An A layer comprising Al_(x)Ga_(1−x)N having a thickness of 25 Å and a Blayer comprising Al_(y)Ga_(1−y)N (x>y) having a thickness of 25 Å arelaminated alternately 30 times to form a n-side light guiding layer of asuperlattice multi-layered structure having a thickness of 0.15 μm. Uponthis, by changing an Al ratio x in an A layer from 0.05 to 0.03 andrendering an Al ratio y in a B layer constant at 0.015 with growing, an-side light guiding layer 106 having a gradient composition isprovided. Upon this, in a n-side light guiding layer, at a first regionhaving a thickness of 50 nm, an A layer and a B layer are both formed bySi doping and, at a remaining region having a thickness of 0.1 μm (aregion on an active layer side having a thickness or 0.1 μm), only an Alayer is doped with Si and a B layer is formed undoped in a modifieddoping manner. Here, a n-side light guiding layer has smaller band gapenergy and Al crystal mixing ratio than those of a first barrier layerin an active layer, resulting in a first nitride semiconductor layer.

(P-Side Light Guiding Layer 109 (Second Light Guiding Layer 29))

an A layer comprising Al_(x)Ga_(1−x)N having a thickness of 25 Å and a Blayer comprising Al_(y)Ga_(1−y)N (x>y) are laminated alternately 30times to form a p-side light guiding layer 109 of a superlatticemulti-layered structure having a thickness of 0.15 μm. Upon this, bychanging an Al ratio x in an A layer from 0.03 to 0.05 and rendering anAl ratio y in a B layer constant at 0.015 with growing, a p-side lightguiding layer 109 having a gradient composition is provided in which theband gap energy grows greater and an Al average ratio grows greater asgoing away from an active layer and a p-side electron confining layer108. Here, in a p-side light guiding layer, at a first region having athickness of 0.1 μm (region on an active layer side having a thicknessof 0.1 μm), only an A layer is doped with Mg and a B layer is formedundoped, at a remaining region having a thickness of 50 nm, both an Alayer and B layer are doped with Mg. Here, in a multi-layered membranein which an A layer and a B layer are laminated periodically, only oneof layers has a gradient composition. Alternatively, both layers mayhave a gradient composition.

The resulting laser device has approximately the same Al average ratioas compared with Example 13. However, the crystallizing property becomesbetter due to a superlattice structure and the device property isimproved. On the other hand, since an undoped region in a light guidinglayer is smaller as compared with Example 13, there is a tendency thatthe light loss is increased and a threshold current density is slightlyincreased.

COMPARATIVE EXAMPLE 1

A laser device is manufactured which has a structure in which a lightguiding layer has the greater band gap energy than that of a well layerand a barrier layer in an active layer as shown in FIG. 14A, and an Alcrystal mixing ratio in a light guiding layer is greater than that of anactive layer and, further, an Al crystal mixing ratio in a claddinglayer is greater than that of a light guiding layer as shown in FIG. 16.According to the same manner as that of Example 9 except that a lightguiding layer and a cladding layer are as described below in Example 9,a laser device is obtained.

(N-Side Cladding Layer [Lower Cladding Layer 25])

An A layer comprising Si-doped n-type Al_(0.17)Ga_(0.83)N having athickness of 25 Å and a B layer comprising Si-doped N-typeAl_(0.20)Ga_(0.75)N having a thickness of 25 Å are laminated alternately100 times to form a superlattice multi-layered cladding layer.

(N-Side Light Guiding Layer [Lower Light Guiding Layer 26])

Undoped Al_(0.17)Ga_(0.8)N is formed at a thickness of 0.15 μm.

(P-Side Light Guiding Layer [Upper Light Guiding Layer 29)]

Undoped Al_(0.17)Ga_(0.8)N is formed at a thickness of 0.15 μm.

(P-Side Cladding Layer [Upper Cladding Layer 30])

An A layer comprising Mg-doped p-type Al_(0.2)Ga_(0.8)N having athickness of 25 Å and a B layer comprising Mg-doped p-typeAl_(0.25)Ga_(0.75)N having a thickness of 25 Å are repeatedly laminatedalternately 100 times to form a superlattice multi-layered claddinglayer.

Crack is produced throughout the resulting laser device and the laserdevice can not operate. In addition, even when the device can operate, aleaking current due to deterioration of the crystallizing property isproduced much and, thus, laser oscillation can not be obtained.

EXAMPLE 15

A light emitting device 200 of the present invention will be explainedby referring to FIGS. 15A and 15B. Here, as shown by 200 b, a lightemitting device having a structure in which one pair of a positiveelectrode and a negative electrode are provided on the same side of asubstrate is manufactured.

A substrate 201 composed of sapphire (C plane) is set in a reactor forMOVPE, and a temperature of a substrate is risen to 1050° C. whilehydrogen is flowing, to clean a substrate.

Buffering layer (not shown): subsequently, a low temperature-growingbuffering layer comprising GaN is grown on a substrate 1 at a thicknessof about 100 Å. This low temperature-growing layer is grown at a lowertemperature as compared with a layer to be grown next, whereby, latticeincompatibility with a substrate is alleviated, and this low temperaturegrowing can be omitted depending upon a kind of substrate.

Ground layer (not shown): after growth of a buffering layer, an undopedGaN layer is grown at a thickness of 1.5 μm at a temperature of 1050° C.This layer is grown as an undoped layer and serves as a ground layer fora device structure to be formed thereon and functions as a growingsubstrate.

N-type contact layer 202: subsequently, a n-type contact layer (electroninjecting layer) 202 comprising Al_(0.05)Ga_(0.9) ₅N doped with Si at4.5×10¹⁸/cm³ is grown at a thickness of 2 μm at 1050° C. Here, a n-sidecontact layer 202 serves as a first nitride semiconductor layer.

Active layer 203: a barrier layer (first barrier layer 2A) comprisingundoped Al_(0.15)Ga_(0.85)N is grown at a thickness of 100 Å andsubsequently, a well layer comprising undoped Al_(0.05)Ga_(0.95)N isgrown at a thickness of 30 Å. Subsequently, an internal barrier layer(not shown) comprising Al_(0.1)Ga_(0.9)N having a thickness of 30 Å isgrown, four layers of a well layer (not shown) and three layers of aninternal barrier layer (not shown) are laminated alternately and,finally, Al_(0.15)Ga_(0.85)N as a second barrier layer 2 b is grown at athickness of 40 Å to grow an active layer 203 of a multiple quantum wellstructure having a total thickness of 380 Å. As shown in FIG. 14B, thisactive layer has a structure in which an internal barrier layer (such as2 b) having smaller Al crystal mixing ratio and band gap energy thanthose of a first barrier layer 2 a and a second barrier layer 2 b isformed.

P-side cladding layer 204: An A layer 204 comprising undopedAl_(0.2)Ga_(0.8)N is grown at a thickness of 40 Å and subsequently, a Blayer 205 comprising Al_(0.05)Ga_(0.95)N doped with Mg at 5×10¹⁹/cm³ isgrown at a thickness of 25 Å. These procedures are repeated to laminateeach five layers of an A layer and a B layer in this order and, finally,an A layer is grown at a thickness of 40 Å to obtain a multi-layeredsuperlattice structure. Thus, a p-side multi-layered cladding layer 204comprising such the multi-layered membrane is grown at a thickness of365 Å. Upon this, a first B layer has smaller band gap energy than thatof a second barrier layer, and serves as a second nitride layer having asmall Al crystal mixing ratio.

P-side contact layer 205: Subsequently, a p-type contact layer 205comprising GaN doped with Mg at 1×10²⁰/cm³ is grown at a thickness of200 Å.

After completion of the reaction, a temperature is dropped to roomtemperature, and a wafer is annealed at 700° C. in the nitrogenatmosphere in a reactor, to make a p-type layer less resistant.

After annealing, a wafer is removed from a reactor, a mask having aprescribed shape is formed on the surface of an uppermost p-side contactlayer 205, etching is performed from a p-side contact layer 205 with RIE(reactive ion etching) apparatus to expose the surface of a n-sidecontact layer 202 as shown in FIG. 15A.

After etching, a light-transmittable p-electrode 206 containing Ni andAu having a thickness of 200 Å is formed on almost all the side of anuppermost p-side contact layer 205, and a p-pad electrode (not shown)comprising Au for bonding is formed on the p-electrode 206 at athickness of 0.5 μm. On the other hand, a n-electrode 207 containing Wand Al is formed on the surface of a n-side contact layer 202 exposed byetching, to obtain LED device.

This LED device indicates ultraviolet emitting at a wavelength of 355nm. In particular, by provision of the aforementioned first nitridesemiconductor layer, an active layer can be formed with the bettercrystallizing property and a light emitting device excellent in theemitting property is obtained.

INDUSTRIAL APPLICABILITY

Effect of the Invention

A nitride semiconductor device of the present invention can afford anactive layer and a waveguide structure which can be laser-oscillated ata short wavelength region of 380 nm or shorter. In particular, in a welllayer of InAlGaN, by adopting an In crystal mixing ratio in a range of0.02 to 0.05, preferably 0.03 to 0.05 and changing an Al ratio to form aforbidden band having a desired emitting wavelength, whereby, a lightemitting device and a laser device having a short wavelength region canbe obtained and a device excellent in the internal quantum efficacy andthe emitting efficacy can be obtained.

In addition, a nitride semiconductor device of the present invention canafford a light emitting device and a laser device of a shorterwavelength of 375 nm or shorter which have a low threshold current.Therefore, a light emitting diode in combination with a prescribedfluorescent body can provide a substitute for a fluorescent lamp. On theother hand, a laser device shows excellent FWHM and can afford excellentresolution and, as a result, it is useful as a photolithography device.

1. A nitride semiconductor device comprising an active layer providedbetween a first electrically conductive type of layer and a secondelectrically conductive type of layer, wherein said active layer has aquantum well structure including at least a well layer formed of anitride semiconductor containing In and Al, and a barrier layer formedof a nitride semiconductor containing Al, wherein an emitting wavelengthof said active layer is 380 nm or shorter, and said device has a laserdevice structure in which said first electrically conductive type oflayer has a first light guiding layer, said second electricallyconductive type of layer has a second light guiding layer, and saidactive layer is provided between said first light guiding layer and saidsecond light guiding layer, and the band gap energies E_(g) of both saidfirst light guiding layer and said second light guiding layer aregreater than the photon energy E_(p) of laser light by 0.05 eV orgreater (E_(g)−E_(p)≧0.05 eV).
 2. The nitride semiconductor deviceaccording to claim 1, wherein said well layer is formed ofAl_(x)In_(y)Ga_(1−x−y)N (0<x≦1, 0<y≦1, x+y<1) and said barrier layer isformed of Al_(u)In_(v)Ga_(1−u−v)N(0<u≦1, 0≦v≦1, u+v<1).
 3. The nitridesemiconductor device according to claim 2, wherein a thickness of saidwell layer is smaller than that of the barrier layer.
 4. The nitridesemiconductor device according to claim 3, wherein an In compositionratio y in said well layer is in a range of not less than 0.02 and notgreater than 0.05.
 5. The nitride semiconductor device according toclaim 3, wherein an In composition ratio y in said well layer is in arange of not less than 0.03 and not greater than 0.05.
 6. The nitridesemiconductor device according to claim 1, wherein said first lightguiding layer and/or said second light guiding layer are formed ofAl_(x)Ga_(1−x)N (0≦x≦1).
 7. The nitride semiconductor device accordingto claim 4, wherein said active layer emitts light of wavelength of 380nm or shorter, and said first electrically conductive type of layerand/or said second electrically conductive type of layer are formed ofAl_(x)Ga_(1−x)N (0<x≦1).
 8. A nitride semiconductor device having anactive layer between a first electrically conductive type of layer and asecond electrically conductive type of layer, wherein said active layerhas a quantum well structure including at least a well layer formed of anitride semiconductor containing Al, and a first barrier layer formed ofa nitride semiconductor having a band gap energy larger than that of thewell layer in a side near to the first electrically conductive type oflayer from the well layer, and said first electrically conductive typeof layer includes a first nitride semiconductor layer having a band gapenergy smaller than that of said first barrier layer, and said firstnitride semiconductor layer is provided near said first barrier layer,wherein said device is operable to oscillate light of wavelength of 375nm or shorter, an Al_(x)Ga_(1−x)N quantum well layer (x≧0) is providedbetween barrier layers which are formed of Al_(y)In_(z)Ga_(1−y−z)N(z≧0), and a band gap energy E_(w) of the well layer is larger than aband gap energy E_(b) of the barrier layer by 0.2 eV or greater, andsaid device has a SCH structure in which a light guiding layer and acladding layer are provided apart from each other, and the band gapenergy E_(g) of the guiding layer is greater than the photon energyE_(p) at oscillation by 0.05 eV.
 9. The nitride semiconductor deviceaccording to claim 8, wherein said first barrier is arranged in a sidenearest to the first electrically conductive type of layer within theactive layer, and said first nitride semiconductor layer is contactedwith the active layer.
 10. The nitride semiconductor device according toclaim 9, wherein said first electrically conductive type of layer is ofn-type, and said second electrically conductive type of layer is ofp-type.
 11. The nitride semiconductor device according to claim 10,wherein an Al crystal mixing ratio X_(B1) in said first barrier layerand an Al crystal mixing ratio X_(w) in the well layer satisfy thefollowing relation: X_(B1)−X_(W)≧0.05.
 12. The nitride semiconductordevice according to claim 11, wherein a thickness of said first barrierlayer is 30 Å or greater.
 13. The nitride semiconductor device accordingto claim 12, wherein a waveguide is constructed of a pair of lightguiding layers and an active layer therebetween, and the light guidinglayer is provided in the first electrically conductive type of layer,and has said first nitride semiconductor layer.
 14. The nitridesemiconductor device according to claim 8, wherein a thickness of thewell layer is 300 Å or smaller.
 15. The nitride semiconductor deviceaccording to claim 14, wherein a thickness of the barrier layer is 300 Åor smaller.
 16. The nitride semiconductor device according to claim 8,wherein the light guiding layer comprises anAl_(a)Ga_(1−a)N/Al_(b)Ga_(1−b)N (a≠b) superlattice layers.
 17. Thenitride semiconductor device according to claim 16, wherein the claddinglayer comprises an Al_(c)Ga_(1−c)N/Al_(d)Ga_(1−d)N (c≠d) superlatticelayers, and the band gap energy E_(c) of the cladding layer is greaterthan that of the light guiding layer.
 18. The nitride semiconductordevice according to claim 15, wherein said device has a GRIN structurein which a light confining layer with a stepwise varying refractiveindex is formed outside the quantum well layer, and non-doped layers areprovided above and below the active layer.
 19. A nitride semiconductordevice having an active layer between a first electrically conductivetype of layer and a second electrically conductive type of layer,wherein said active layer has a quantum well structure including atleast a well layer formed of a nitride semiconductor containing Al, anda first barrier layer formed of a nitride semiconductor having a bandgap energy larger than that of the well layer in a side near to thefirst electrically conductive type of layer from the well layer, andsaid first electrically conductive type of layer includes a firstnitride semiconductor layer having a band gap energy smaller than thatof said first barrier layer, and said first nitride semiconductor layeris provided near said first barrier layer, wherein said device isoperable to oscillate light of wavelength of 380 nm or shorter, and anactive layer is provided between a first electrically conductive type oflayer and a second electrically conductive type of layer, and saidactive layer has a quantum well structure including at least a welllayer formed of a nitride semiconductor containing In and Al, and abarrier layer formed of a nitride semiconductor containing Al, and saiddevice has a laser device structure in which said first electricallyconductive type of layer has a first light guiding layer, said secondelectrically conductive type of layer has a second light guiding layer,and said active layer is provided between said first light guiding layerand said second light guiding layer, and the band gap energies E_(g) ofboth said first light guiding layer and said second light guiding layerare greater than the photon energy E_(p) of laser light by 0.05 eV orgreater (E_(g)−E_(p)0.05 eV).
 20. The nitride semiconductor deviceaccording to claim 19, wherein said well layer is formed ofAl_(x)In_(y)Ga_(1−x−y)N (0<x≦1, 0<y≦1, x+y<1) and said barrier layer isformed of Al_(u)In_(v)Ga_(1−u−v)N(0<u≦1, 0≦v≦1, u+v<1).
 21. The nitridesemiconductor device according to claim 20, wherein a thickness of saidwell layer is smaller than that of the barrier layer.
 22. The nitridesemiconductor device according to claim 21, wherein an In compositionratio y in said well layer is in a range of not less than 0.02 and notgreater than 0.05.
 23. The nitride semiconductor device according toclaim 21, wherein an In composition ratio y in said well layer is in arange of not less than 0.03 and not greater than 0.05.
 24. The nitridesemiconductor device according to claim 19, wherein said first lightguiding layer and/or said second light guiding layer are formed ofAl_(x)Ga_(1−x)N (0≦x≦1).
 25. The nitride semiconductor device accordingto claim 22, wherein said active layer emits light of wavelength of 380nm or shorter, and said first electrically conductive type of layerand/or said second electrically conductive type of layer are formed ofAl_(x)Ga_(1−x)N (0<x≦1).
 26. The nitride semiconductor device accordingto claim 1, wherein the Al composition ratio x and the In compositionratio y in said well layer satisfy the following relation: x≧y.
 27. Thenitride semiconductor device according to claim 19, wherein the Alcomposition ratio x and the In composition ratio y in said well layersatisfy the following relation: x≧y.